Switching 454 cab out for 350 cab

[1968 Pontiac]

My 1989 K3500 Cheyenne with 454 has an excellent drive train with most of it rebuilt by the previous owner, a nieghbour from down the road, but a poor body with many dings and large dents due to this being a work truck in a previous lifetime. The previous owner decided to go 4x4ing with a Samurai in lieu of the Chev, so I bought the Chev for cheap.

Anyway, I've now bought a 1989 C2500 GMC Sierra SLE with excellent body and 350 engine. I'm going to switch the body from the GMC to the Chev, but I'm wondering if I need to switch the wiring harness also, or can I leave that alone. There were two different computers common to both trucks, so as long as I have the matching computer, that part of the harness shouldn't be a problem. Both trucks also have th400 trannies. If it makes any difference, the C2500 is the heavy 3/4 ton with eight bolt rims. I'm going to be parking the C2500 this weekend at our summer property two hours away to be with the K3500. If I have time, I'll check the computer in the one ton. Both of the trucks are RC/LB.

Any help would be appreciated, but this will be a spring project which I'm just trying to get organized now so I can hopefully do the switch without too many problems.

[2000CAYukon]

I have the factory manuals for 1990 and the computers are the same for 350 or 454. C or K, it does not matter. The Chips (or the programs on the Chip) is different.

You can verify this by pulling both ECMs and looking at the part numbers. The harness connectors at the ECMs should be the same.

I think the engine connections are the same between the 454 and 350 so you can probably use the harness that is already in the C2500. The fact that the tranny is the same makes things real easy.

Sounds like a fun projects and you should have a real nice truck when you are done.

//2000CAYukon

[1968 Pontiac]

Thanks for the info and insiration Yuk!! Actually, what I was going to do, and I was hoping the computer connectors would be the same, is leave the engine harness on the 454, just unplug it at the firewall. I'll also disconnect all the 454 power connectors at the buss bar on the firewall. If I'm lucky the 4wd t-case will take it's power from the buss bar, & be grounded somewhere on the firewall; then I only have to disconnect the wires/harness that go to the floor shifter.