4.3l to 5.7l TBI

I have a 91 Sonoma SLE 2WD that I bought with a 350 TBI already in it. The previous owner didn't know alot about it, but did tell me that when it was converted that no wiring was spliced and the original harness and ECM was still original. I have noticed the engine seems to be starved for fuel and I only get @15mpg. I wanted to know do I need to change the ECM to a 350 TBI and will it match the harness?

Try changing the injector in the throttle body. or the ecu from a full size car or truck. Not real sure about this answer i'm just goin off of givin advice from another member