1968 Firebird muscle engine?

Okay, so if i decide to add a muscle engine in, and the max price is 5g, is it good? Not looking for a engine too strong because it is going to cost too much and not an engine to weak. Looking for an engine in the between, around maybe 500 if possible. Is that a good price or not? If it is, what engine would you prefer? If not, how much would it cost for a good engine, and what engine is it? Specs if possible. Or should i just rebuild it as stock? Not for selling purposes, personal use only.