The Stovebolt Site Forums The Parking Lot Big Bolts SM420 - Swap or Gear Vendor Overdrive

Don't use gear vendors. It's way overpriced for what it is, and it won't hold up if you are actually going to be hauling like you say.

The simplest old school swap will probably be a clark vo transmission which will be a 5 speed overdrive. They are kind of rare, but they turn up every now and then. I'd like to hear from someone who has experience with the older ones because I'm curious if they will bolt to an sm420 bell.

One obvious choice is an nv4500. You can find these in '90's to 2007 chevy pickups, and they will bolt right up to a small block, but you will need to remove your bellhousing crossmember. If you want to keep your bellhousing crossmember, you will need an adapter plate to mate the transmission to the original bell housing. I did my swap this way nv4500 with sm420 bellhousing

Another option is the spicer 3053a which is an overdrive 5 speed. I've seen those transmissions adapted to chevy motors. Again, bellhousing crossmember removal

I just posted a transmission in the parts for sale forum. It's an eaton fuller fs4005b with all the factory parts to bolt it to a big block. It would be a good choice except for the lack of an overdrive and bellhousing crossmember removal

If you want an overdrive fuller transmission (apart from some of the newer units they've been putting out), you need to step up to a 6 speed, but that makes the weight go up as well. Their smaller 5 speeds weigh 230lbs which is in the nv4500 range as far as weight goes. The 6 speed overdrives weigh around 350lbs which is in nv5600 territory, but I would choose that over an nv5600 any day.

Another option would be to try and adapt a g56 or zf6 to your motor. Those are both 6 speed overdrive transmissions that are reasonably light.

If you are still reading, haha, I would suggest swapping your sbc for a 12 valve cummins (you should have room in there), and mating it to an fso6406 or fso8406. If I couldn't find a decent fso trans, my second choice would be a g56. Both of those transmissions will bolt to the cummins with factory parts, and you would end up with a simple, reliable, and strong drivetrain.