86 GMC C1500 V-6 to V-8 Swap

[ZRX12UTAH]

We swapped a siezed 4.3L V-6 to a 350. The engine is completely installed, and everything is connected. Problem: It wont run steady.
We installed a brand new GM HEI V-8 distributor, We sat the distributor in with the rotor sitting on No# 1 post, Verified and adjusted the ignition timing With the timing light on No# 1.
It cuts out and runs rough, A freind said that there was computer on the 1986 C1500, and to swap it with a V-8 model? I looked onto of the Heater Housing and there is a small silver box there. But on the HEI there is only a red 12v wire, and a Tach wire. Am I chasing a ghost here.
I have checked all the ground wires.

[GOLIATHGMC]

Lets start with some basics.
Was the 4.3 Carb, TBI, or MPI? Was the original distributor an HEI? I am going to assume that the v6 was at least TBI and computer controlled (that little silver box under the dash over the heater box). That computer will have to be swapped out for one that is made for a V8. The rest of the wiring harness should connect right up (the 4.3 is a 350 chevy minus 2 cylinders). As for the distributor, I cant see how it would matter - a standard distributor has the same connections. Look into changing the computer (Engine Control Module or ECM).
George
REBEL Motorsports