Swapping out a 207 for a more modern motor is not an easy task. If you do a search you will find some helpful hints. A modern 235 from 1954 up would give you a full pressure motor and plenty of power. You could also put a 216 valve cover on it to make it look closer to the period of your truck. If looks aren't important, a 250 would be a more modern and rugged motor.
