Just wanted to find out why the the a 1986 360 GLT runs 80amp Alternator and the rest of the 340's run a 55Amp?

What would happen if a 55 amp Alternator was used instead of an 80 amp alternator

Sounds like you have an odd alternator, according to my info 360 was 55A and 340 mostly 50A. Not much different in power output really, main difference was the drive pulleys have different sizes and offsets, 360 can use 2 drive belts. 360 had a bigger starter and maybe a bit more equipment otherwise doesn't matter. People fitted big alternators in the 2000's for silly power amps.

It is a B200E engine and I'm only assuming it's a 80 amp one. I was looking at buying a 55 Amp one because they are loads cheaper. I'll Check