I believe you have a dropout style differential. Go to a driveshaft shop and tell them what you have and you want a driveshaft with a slip yoke for a sm420. They will advise and tell you what measurement they need and build it for you. One piece will work fine.

Option: there will be 55-59 trucks with dropout diff and SM420 to salvage a slip yoke driveshaft from. 57 is 114" WB and 51 is 116" WB. Shortening or lengthening an existing D/S may be cheaper.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn