Computer Compatability?

:banghead: :feedback: I bought a 91 Sonoma that had a Chevy TBI 350 put in. The previous owner was unaware of the year of the engine, but knew that the engine was hooked up to the 4.3 wiring and computer with little if no cutting and splicing. Now, the truck runs, and strong, but, it does not run properly. I believe the engine is being starved for fuel due to the wrong computer. Is this right and what can I do about this without knowing the year of the engine? Also, it runs at a ludicrous rpm while driving 75 mph (3500rpm). Can this be easily fixed by changing the gear ratio in the rear end? It has the 700R4 for a trans. Thank you to anyone who can help, especially in Michigan.