There were variations in the engine computers and the ignitors over the years. I think they changed almost every year from '85 to '88 and then (verify this first) '89 - '92 were the same, then '93 - '95 were the same.

If you want to run that '94 motor in your '85 you'll probably need an engine wiring harness, a chassis wiring harness and the correct ECU (engine computer). You'll basically have to rewire your whole truck or do a lot of splicing of your old chassis harness. The '94 has different O2 sensor wiring and needs a VSS signal from the transmission (if I remember correctly). -- Matt