I see what you are saying, but how many of us have ever said, "Wow, why would he want to stuff a 426 Hemi in his 67 Barracuda?" or "Nah, I don't want any more HP because I am not going to race my car". :rofl: :rofl: :rofl:
Seriously though, I think you can put in whatever kind of an engine you want, but I am pretty sure you'll need to swap out the transmission. The old Powerflite will likely not tolerate the power. I'd consider a modern 5 or 6 speed Automatic overdrive. How cool would it be to be able to smoke your tires AND get 25 MPG on the highway.