250 to 402 swap question
 

I know it has been asked before, and I have searched but I cannot find the answer. A little background: 1970 SWB C/10, has 250 inline 6 now with TH400.
I bought all the parts to make the swap, I believe. The question I have is this:
can I use my original one-piece driveshaft? I know someone has done this swap and has the answer, but all I can find is opinions and suggestions.

Thanks,
Mark

Re: 250 to 402 swap question
 

I don't know for sure, but I don't think so. A BB has to be moved back so the crank pulley doesn't hit the front crossmember. When you do that, the driveshaft has to get shorter. A 3-sp, car 4-sp, TH350, and Powerglide are all the same length, a TH400 is longer. I think you can shorten a 1-pc shaft to get there, but don't think you can use the original.

Now with all that said, why would you want a BB with todays fuel prices? Unless you already have all the mounts, pulleys, brackets, manifolds, radiator, etc. this swap will get quite expensive too.

Re: 250 to 402 swap question
 

This truck was a daily driver for me for over 10 years. I have always wanted to have a big block but the opportunity never really presented itself. Now that I am older and have a little more "disposable income" (if there is such a thing, lol), I decided it's time. I have been collecting parts to restore the truck basically since I have owned it so that is the plan for it. I came across a good deal on a stock 402 out of a 71 motorhome with low mileage that had a blown head gasket. So I put rings, bearings, and a mild cam in it and it will be a part-time driver now, so fuel mileage is no longer the goal. The 250 didn't really get that good of fuel mileage anyway with a direct transmission and 3.73 gears. A gear change to 3.42 is also in the plans. I have been collecting the conversion parts to make the swap since I acquired the engine 2 years ago and I have everything now except the driveshaft. I guess I will have the original cut down since I don't want to go with a 2-piece. This is a basically stock motor so it will not be making a ton of HP. Thanks for the info.

Re: 250 to 402 swap question
 

Good news, bad news.

Possible good news: There are 3 bolt holes on the top of the frame rail for the brackets that connect the engine mounts. If the engine it has now is in the rear two holes then it is in the right position for a big block so the trans can stay where it is.

Possible bad news: Only catch, you need big block engine mount brackets (perches) made for a big block for exhaust clearance. They need to look like these:

http://www.ebay.com/itm/NOS-67-71-Ch...8263d3&vxp=mtr

You can also use 72 parts, however they use different mounts and are more rare/expensive usually.

Re: 250 to 402 swap question
 

I have the towers, mounts, radiator, shroud, heat shield, shift linkage. Is there anything else I will need?

Thanks,
Mark

Re: 250 to 402 swap question
 

Im doing the same thing who cares about gas milage?I have no payments the truck is paid for so fun fun fun!

Re: 250 to 402 swap question
 

You are going to need to get the engine and trans in there and then measure and get a driveline shop to shorten your driveshaft. No big deal.

Re: 250 to 402 swap question
 

Tom, thanks for the info. Apparently the big block SWB trucks used a 2-pc shaft from the factory, is that correct?

Thanks,
Mark

Re: 250 to 402 swap question
 

I have now completed the swap. I had a local driveshaft shop shorten and balance the shaft. I have run it up to 4000 RPM with no issues. Not going to be doing any racing with it so high RPM stress will not be an issue.

Re: 250 to 402 swap question
 

Congrats!Mine is done also 402 5 speed tremec ford 9 inch

Re: 250 to 402 swap question
 

Sounds like a nice setup.

Re: 250 to 402 swap question
 

It is but i have to do a 4 link or traction bars it wheel hops bad !

Re: 250 to 402 swap question
 

The RPM of the motor wouldn't necessarily have anything to do with driveshaft vibration. MPH would be the test.

Re: 250 to 402 swap question
 

Had it up to 100 today and it was smooth. No vibration.