About three people would notice if you put a 308 in it and left the 253 stickers on the air filter lid anyway. Finding a good running motor is what you are after though. I would buy whichever motor that was in the best condition. Rebuilding a v8 properly would have to be up in the two to three thousand dollar price now? It might be more. 15 years since I did one.
I don't know if you can still get exchange motors.