Drive Shaft Nuts

Why are you afraid to take more than one wrench? just throw your toolbox in the trunk/back of the truck and you'll have all bases covered.
What rear end does it have? Straps or U bolts on the U joints? Straps are usually 3/8 head size, although I've seen service kits that have reduced head size bolts in the 5/16 range, or allen screws; so it could have those... If it's U bolts probably a 7/16 or 3/8 wrench. After more than 50 years, who knows what you'll find under there.
And don't forget to take a drain pan and a spare yoke along to plug the transmission tailshaft with, or you'll be puking transmission fluid all over the guy's driveway.