old school overdrive
Elder User
Joined: Jan 2009
Posts: 741
Likes: 8
From: Western N.C.
Most of the five speeds in gas powered medium duty trucks (if not all) are directs, not overdrives.
They are also much heavier and much larger, have larger yokes, and will require some reinforcing/ fabbing of the trans crossmember. I have a Fuller five speed direct in my '62 International, and it is at least twice as big as an NP435. It also tips the scales at over 400 pounds.
Your gains would be slim to nil in exchange for all that work and expense. I suppose theoretically that you could use one from behind a 370 or 429, as they have the same bellhousing pattern. If I was going to attempt a swap like this, I'd take the engine and trans as a unit and swap them both in. That way I could be assured that everything fit and worked together properly rather than hunting all over creation for parts that may or may not exist. The bellhousing itself would probably not fit in our pickup frames without extensive modification. Not to mention matching up clutch parts, linkages, hydraulics, etc.
But that's just me. I think you'd be much better off finding a ZF 5 speed from behind a 460 rather than messing with this idea. The ZF is a popular swap and there is a lot of info and support on this forum for it.
They are also much heavier and much larger, have larger yokes, and will require some reinforcing/ fabbing of the trans crossmember. I have a Fuller five speed direct in my '62 International, and it is at least twice as big as an NP435. It also tips the scales at over 400 pounds.
Your gains would be slim to nil in exchange for all that work and expense. I suppose theoretically that you could use one from behind a 370 or 429, as they have the same bellhousing pattern. If I was going to attempt a swap like this, I'd take the engine and trans as a unit and swap them both in. That way I could be assured that everything fit and worked together properly rather than hunting all over creation for parts that may or may not exist. The bellhousing itself would probably not fit in our pickup frames without extensive modification. Not to mention matching up clutch parts, linkages, hydraulics, etc.
But that's just me. I think you'd be much better off finding a ZF 5 speed from behind a 460 rather than messing with this idea. The ZF is a popular swap and there is a lot of info and support on this forum for it.
New User
Joined: Jan 2007
Posts: 20
Likes: 0
From: Near the 100th Meridian
Years ago the father of a friend of mine put a 6031 spicer auxiliary transmission behind the stock 4 spd. in a 2 WD GM 3/4 ton he had. I was working at an International Harvestor dealership at the time and ordered him the remote shifter tower and a bunch of other small stuff to complete the install. He used an extra crossmember from another GM truck and when he was done it looked pretty close to a factory install. It wasn't cheap at that time because he had to make up two driveshafts plus all the other fabrication. One driveshaft went between the main and auxiliary transmissions and the second one had to be shortened between the auxiliary transmission and the differential. He also had to make the remote shifter linkage to the auxiliary transmission as well. I don't think he did it to improve mileage, but he did it to have more gear selection because of hauling a large fifth wheel trailer. If I remember the 6031 was also a 1.00 direct in third. The people at the GM dealership where he bought the truck thought he was crazy. But it worked and this modification was still in the truck when he sold it many years later.
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