I would say the 5.7 with an overdrive trans, preferably auto.
I have a 64 wagon, 318 poly 2 barrel pushbutton. I always think how kool it would be to have a bb 4 speed wagon, but that would not be a good fit.
Right now, it's reliable, great on gas, comfortable, power steering, brakes, seat, quiet, even with headers, and in a pinch anyone can drive it.
For trips, everyone likes it because of all those things, and you can actually have a conversation on the highway without screaming at each other.
A modern drivetrain would bring it to the next level, but not needed.