If i did an engine swap from a 2.8 to a 305 would i have to change the dash wiring harness or just the brain?

 

Usually the dash harness is left alone unless updating from early wiring to newer. Often Auto to T5 guys keep the stock dash harness and change a few things. The main issue is matching your engine harness and dash harness trans type. Not doing so will send your brake switch and rev lights to dead ends. They can be fixed, but are a bit tricky

Im not sure what type of 305 you're putting in so Ill go both ways

EFI
This requires a donor harness for the V8. Your 89 is a specific year so whatever donor you get needs to be 89 or changes will be required

Carb
A bit easier, you can reuse some of your V6 harness to retain gauge functions
You can modify just what you need to get it running by a few wire changes, but to do it right you need to cut out alot of the harness
Read
https://www.thirdgen.org/forums/ltx-...p-3rd-gen.html
89 pinouts can be found here
https://www.thirdgen.org/forums/ltx-...formation.html