I want to put a 1990 chevy 350 and trans into my 77 pontiac grand prix. What issues should I be on the lookout for?

 

Difference in the bellhousing, the 90 model has a different bolt pattern, the tranny for the older engine will not have a wiring harness and the ECM for the 90 engine and transmission sensors. cruise control and other electronic devices in the original car are different from the donor you might also have to transfer the body control module. Fuel pump the older vehicle has a manual fuel pump whereas the newer engine setup has an electric fuel pump, there might be others but that is all I can think of at this time.