Swap 454/4L80 from 92 C2500 into 88-98 C1500

[68 TT]

I did the same swap. 89 C1500 cab & 91 K1500 chassis with 94 K2500 donor TBI 454/4L80E/NP241. I ended up only needing to modify the main PCM wire harness through firewall grommet. The hole in the firewall where the wire harness passes through in the 89's cab was a different shape than the 94 donor vehicle.

It was either cut the section of metal out of the 94 and graft it into the 89 cab or remove the bulkhead shroud from the wire harness from the 89 and fit it to the 94 harness so it would fit. I chose to try the bulkhead shroud swap. It sucked but I got it done and it worked but was a very tight fit for all the extra wires in the newer harness. If I was to do it again I would opt for the cut and weld route instead.

You could avoid this entirely by getting a C1500 truck with the newer firewall design to start your build from. If you like the classic dash design I would go with a 94 as you also get a better grille design that won't fit on the older core supports and the A/C system is already 134a from the factory as well as having the max A/C feature the older ones don't.

I ended up pulling the 4L80E/NP241 and installing a T-400 due to other problems but it all cleared the C1500 / K1500 parts just fine.

Not really related but kind of. The 89 & 94 cruise control systems use mostly the same parts but some stuff is slightly different and the wiring is slightly different even through the common wire diagrams show it all as the same for 88-94. I didn't notice the differences until it was too late to properly document them. Swap the engine donor wire harness, steering column, pedal assembly and cruise control module and all connected wiring or the system may not function properly. I thought I had mine done correctly with a mix of parts that were in the best condition but it is not fully functional. It will engage and maintain speed but won't cancel when you step on the brakes. I need to spend some time at the wrecking yard with a complete 94 and do a forensic analysis of the components and wiring colors to see what I have in my assembly that is incompatible.