Measure the length retracted, extend and remeasure that will give you the stoke you need.
Measure the shaft diameter when extended
Measure the cylinder diameter on the main tube and look for one thats the next size smaller. Say it measures 4 7/8" its a guess yes but i'd say 4 1/2" leaving it with a 3/16" thick wall IF it measures 5" I'd still guess it at 4 1/2" with a 1/4" wall
Hope that makes sense.
Keep in mind the pressure your system will generate when selecting a cylinder. Sorry i have no idea what that is on yours. I'd guess it to be close to 2200-2500 psi most of the older ford stuff is but again a guess.
Whats the saying, Close only counts in horse shoes and hand grenades lol
Joe