I6 250 swap for 454

[Hair]

1971 GMC C10 2wd with 250 inline.
I am swapping the I6 for a 454.
Do they use the same motor mounts, or do I have to buy mounts for the 454?
Once I get the I6 out, I want to be prepared and stick the 454 back in without having to run around and find motor mounts if I need different ones.

[Shaky]

I think you need new frame brackets also.

[Hair]

Frame brackets, yep, da, thanks, I didn't even think of that... so I need the frame brackets and motor mounts too maybe?

[Shaky]

I've never messed with big blocks, but Rock Auto lists different part number for the 1971 250 and 402 motor mounts.

The I6 and V-8 frame brackets are different. We put a small block on I6 brackets once. It fit but it stuffed the distributor against the firewall. So I imagine the BB brackets will move the engine forward. Point being - if you get BB brackets, you may have to slide your transmission cross member forward too.

I'm not an expert on this. I know there's a difference between small-block and I6 brackets, but I don't know if there's a difference between SB and BB brackets.

Someone here who knows will be along shortly with all the correct info...

[Hair]

Oh man!... surely the tranny won't have to be moved too?

I'm hoping maybe just some new perches is all I need... Motor mounts will be the same, and the new perches will situate the BB to the tranny and firewall as it should be? I'm staying optimistic

I'd say there has to be not less than 100 people on here who have made this swap... maybe they'll chime in.

[Grumpy old man]

Originally Posted by Hair

Oh man!... surely the tranny won't have to be moved too?

I'm hoping maybe just some new perches is all I need... Motor mounts will be the same, and the new perches will situate the BB to the tranny and firewall as it should be? I'm staying optimistic

I'd say there has to be not less than 100 people on here who have made this swap... maybe they'll chime in.

Motor mounts , trans mounts, driveshaft ? Springs , shocks , you can't drop a big block in where a l6 has been living without a ton of upgrades , don't forget all new hoses belts ,radiator etc etc etc. If it was easy or cheap everyone would have one ! Do your homework it's a lot of parts and work with the right tools and equipment

[Hair]

Yeah, I've been gearing-up toward this project for several years now while still driving the truck regularly. I've added BB coil springs and shocks when I rebuilt the front end and added a large aluminum radiator, wired it for HEI, I have all the pulleys and brackets etc., I pulled the tranny (PG) and had it redone about a year ago. I even have the exhaust ready, only using one side right now. I just have to modify it to fit perfect once the BB is in. Everything that I have done to this truck so far I have done it thinking "Big Block" because I knew where I was eventually headed.

But now I'm actually there, so I'd like to be prepared for the mounting situation before I pull the engine out.

[Rich69shortfleet]

That Powerglide is not a great choice to have behind that 454.

[71meangreenc10]

I done this with my 71 283.

If you want it done right, everything has to be moved into the forward position for the frame mounts. Since you are using an PG, moving the trans will be easy (unless you don't have tools to break any rivets) The BB motor perches are different than the small block ones, if I remember correctly, SB may work. The center hole is bigger for BB. Someone else will chime in that has some. You will need to extend the driveshaft. A few smaller details to ad, but I just got back from flying half way around the globe. Shoot me a PM if you have questions.

Thanks,

Smitty

[Willys47]

big block moumt dip for thr exhaust manifolds

[Willys47]

side view for ya

[Hair]

Willy thanks for pics. Now, remove those from your truck and send them to me. When you get them ready pm me for my address

I just got home where I could actually look at it, and the motor mounts on the I6 look totally different too, so I'm now convinced that I need perches AND motor mounts for sure.

Using a tape measure and eyeballing it, the I6 and the BB both measure 16" from center of the mount to the back of the block. I have a good 2" between the I6 and the firewall. It "appears" that it would fit. Will the BB perches move the block forward? Therefore moving everything forward? Is that why I'm going to have to move my tranny forward too?

[71meangreenc10]

Look at the frame holes, the front two are for V8's and the back are for I6 (2wd). You can try to use the 6 holes for the 8. I never did a BB, but I did have to use it initially for my 283 and it worked, but the valve covers, HEI were too close to the firewall for me. Didn't look right for a 2wd in my opinion. 4wd's are a different animal and use the back holes for 8's and use a different crossmember.

Tall valve covers will have issues in the 6 spot, large HEI cap, hits the wall, it did for the SB....

Yes, the perches move it forward just like the SB ones do.

Smitty

[70STOVEBOLT]

You have to use the BB towers but you can use SB mounts with a large washer on the bolt because the BB towers have a larger hole in them. The dive shaft will need shortened and balanced. Mine was shortened to 59.5 inches and I used the front two holes in the frame to mount the towers. I have a SWB truck. If yours is LWB you will just have to measure to see what driveshaft length you will need. I used a square body 3-row radiator and the lower neck is a little low, as it is straight and the radiators for 67-72 have a curved lower neck. I cut a piece of the old radiator hose and put it between the new hose and the frame where it kicks up to keep from wearing a hole in the new hose. Mine is short water pump. You will need a big block shroud also. I did have to move the transmission crossmember to the rear holes. The block lengths are the same from the rear face to the motor mount holes, but the shape of the mounts made it necessary to move the trans crossmember. I have a TH400, I don't know If you would need to move it with the PG. If I think of anything else I will let you know.

[Hair]

OK, "if I want it done right" (and I want it done right) the motor goes forward. I see in Willy's picture, he's in the front holes.

So I'm thinking the procedure goes like this:
1.) Remove I6.
2.) Grind/drill/beat/kick/cuss the rivets out of cross-member/frame, slide tranny forward and stick some bolts in it where the rivets were?
3.) Remove old towers, replace with BB towers & BB mounts
4.) Drop in BB - bolt it down and bolt up converter and tranny.
5.) Find a longer drive shaft and stick it in there.
6.) Then I'll worrying about fans/water pumps/hoses/linkages/headers and all that crap.

That's my current plan anyway. Am I getting it right?

I haven't looked under there at my cross-member yet. Does the frame have extra holes already in it for the cross-member to move forward?

[70STOVEBOLT]

Yes, your game plan sounds correct. There should be an extra set of holes to bolt the crossmember to. My 70 had them anyway, I would think if the holes for the engine are there, the holes for the crossmember are there.

[RickDeVon]

Thanks to all of you for the info ....only I'll be going I250 to 383...700R4...Or LS5.3...in my '71...Hair good luck with your build...

[casper1969]

I did this swap in my 69 gmc original I 6 to a 454. I have the original Th350 though so I did not have to relocate Trans cross member because I used the correct BBC engine stands and mounts for this year. I believe the newer clamp she'll mounts were forward more so I opted for the old style which are very pricy. Then when dropping engine all you need to do is check clearances for distributor . You will then want to change Everything in the engine bay to BBC a good catalog is LMC you can use as reference it will make your swap a breeze but if you want it period correct prepare to drop some coin. Have fun it will be worth it

[70STOVEBOLT]

I also used the 68-71 towers but I still had to move the trans back to the rear set of holes and have the driveshaft shortened. I have no clearance issues with valve covers or HEI distributor cap and the firewall.