I have a 1989 stock truck chevy 1500 step side, thats been sitting in a garage for years, truck is like new, I purchased it from my 66 year old father in law...

My question is, #1) what muffler should I put on this truck (flowmaster etc) and how much is it going to cost me at a shop for install etc.

#2) what are some low cost things I can do to the motor to get some more HP

any input would be great!