My personal preferences would be to either freshen up the 250 and add the turbo if it's in the budget, or put in either a 283 or 305 sbc. With the 250 you have your original engine and decent mileage. I believe the aftermarket intake/exhaust and turbo would both slightly improve mileage and give you a power boost, but I could be wrong. If you swapped it out for a small v8 you would still have a solid, reliable engine but you would have more power at your disposal. Your mileage would drop some but should still be reasonable. I don't know any numbers off the top of my head about power gains etc as I am fairly new to this but those would be my two choices. As far as going to EFI, I would avoid that. EFI is really nice... when it works. I prefer carbs because they are mechanical and I can work on them, I'm not dependent on a computer that way. But that's just my opinion, if you want EFI go for it
