Compare the length of the motors. You may find the '55 motor is a little shorter and has a different water pump arrangement. The shorter motor will fit better in your early frame.
'54 and '55 1st truck motors are both short like the earlier motors, and full pressure oil, making them the easiest swap option with all the good stuff.
I believe you can swap all the shorter parts to the later motor, but you'll want to have that in mind as you go.
