273

There is NOTHING at all wrong with the 273. it is a FINE motor. You would probably be very happy if you just rebuilt a 273. It will provide you with plenty of power for cruising around while giving you decent mileage. If, on the other hand, you are thinking about swapping the motor out, I would go with a 360 for several reasons.
1. The 360 has 87 cubic inches more than the 273. Even with a very mild build, a 360 could easily have 50-60 more HP and torque foot pounds than the 273.
2. If you spend a bunch of money rebuilding a 273 and change your mind for a bigger engine in a few years, you will be luck to sell your 273 for half of what you have in it.