original 3 speed or sm420?

[Mr Big]

My 66 3/4 is down due to a sudden onset of tranny noise. I have yet to pull the original 3sp so we're not sure yet what will be involved to repair it.

Bought this truck from the original owner who kept very detailed maintenance records and the truck is currently completely original, so I'd like to stay with a factory available configuration if we convert.

Was thinking of trying to source an original sm420, shifter, and floor hump to convert to 4sp? Will these parts be difficult to find?

Thanks for the advice/help. Glen in TN.

[XLCOR_65]

I recently converted from the 3 speed to the SM420 and seriously cannot stop grinning as I drive it now. I love the compound gear and also the particular whine it makes. It just feels a lot tougher and more purpose built, in my opinion.

For a quick down and dirty swap, you'll obviously need the trans and high hump, of course, but you'll also need the beefier drive shaft and heavy duty carrier bearing (CaptainFab makes the mount you'll need). I also swapped the columns since I didn't need the 3-speed column anymore, but that isn't completely necessary...you could just get by with removing the shifter and linkages.

Hope this helps. Good luck on whichever you decide!

[Brad54]

If your truck is completely original, you'll need to be aware that swapping to the high hump trans tunnel isn't a bolt-in.
Where the tunnel bolts to the firewall, that flange is spot-welded to the base of the firewall. They use a different flange between high and low hump, so you'll need to break the paint and undercoating to make the conversion.

Just one more thing to keep in mind.

-Brad

[Mr Big]

Originally Posted by XLCOR_65

I recently converted from the 3 speed to the SM420 and seriously cannot stop grinning as I drive it now. I love the compound gear and also the particular whine it makes. It just feels a lot tougher and more purpose built, in my opinion.

For a quick down and dirty swap, you'll obviously need the trans and high hump, of course, but you'll also need the beefier drive shaft and heavy duty carrier bearing (CaptainFab makes the mount you'll need). I also swapped the columns since I didn't need the 3-speed column anymore, but that isn't completely necessary...you could just get by with removing the shifter and linkages.

Hope this helps. Good luck on whichever you decide!

Good info...a 4sp parts truck would be the best situation.

[Mr Big]

Originally Posted by Brad54

If your truck is completely original, you'll need to be aware that swapping to the high hump trans tunnel isn't a bolt-in.
Where the tunnel bolts to the firewall, that flange is spot-welded to the base of the firewall. They use a different flange between high and low hump, so you'll need to break the paint and undercoating to make the conversion.

Just one more thing to keep in mind.

-Brad

Would graft/weld in the center floor cab section from a parts truck work, of could I get away with something less? I'm wondering where the floor contour changes from low hump to high. Glen

[scuderia]

Originally Posted by Mr Big

Would graft/weld in the center floor cab section from a parts truck work, of could I get away with something less? I'm wondering where the floor contour changes from low hump to high. Glen

Pretty straight forward.. simply cut out the original hump leaving a good 1-1/2" of flange for the new one to lay over.

[Mr Big]

Thanks for the good/helpful info. Now to find the parts. Glen

[scuderia]

Originally Posted by Mr Big

Thanks for the good/helpful info. Now to find the parts. Glen

I've got a decent high hump but I don't have the firewall piece.. If interested let me know.

[Mr Big]

Thanks for offering, let me see what I can dig up.

Maybe the whole deal at one spot. Glen

[Isaiah92260]

I would like to bring back this Thread, I've found a really good SM420 for much truck that was pulled out of a running 66, I have original 3 speed too that's tired that I really want to get rid of before I throw in my 283 I still have waiting. My main question is.....

What's the length someone like us need for a Driveshaft? I know that drive shafts for the SM420 60-66 long bed would need to be than the 3 speed would be...