engine swapping 4.3 to 5.7

[elcamino 2]

i have a 91 gmc that someone pulled a 4.3tbi put a 5.7 tbi and didnt change the wiring harness ,he says he used a 4.3 comp. and put a 5.7 prom in it hows that possible. anyway it runs but when it warms up it raises the idle up too about 2000 or 1750 check engine lite is on but gives code 51 im lost, do i have too get a complete wiring harness or can i make the 4.3 harness work