A 1954-1962 high pressure 235 or 261 would be the easiest upgrade.

A 54/55-1st engine would bolt right up, but a 55-2nd to 1962 engine would need a water pump adaptation and maybe a front motor mount adaptation (neither of which are difficult to do). You would use your existing clutch housing, flywheel, and clutch assembly.

Unless you want 12v, there is no need to change from 6v if you upgrade to a 1954-1962 235/261.