‘66 Barracuda

Welcome! That's a really nice looking car you have there. Your question is answered by what YOU want from the car. It certainly looks to be a nice original car so to keep it as it came from the factory is certainly a great option. If you feel you need to have tire roasting capabilities with your car , then an engine upgrade would be the plan. A good running 273 is a very respectable engine and a few minor upgrades can get a little more potential out of it. But, if you feel the need to break the tires loose on demand or plan to race it, the larger engines would probably be a better option. A swap is fairly easy but there are a few details you need to be aware of before doing so, so do some research on this site before jumping into anything.