the assumption that something is safe because it there was no law against it is sorta, well, fuzzy i'm thinking. there were few government agencies in those days overseeing vehicle construction. remember in the post ww2 era, the government had bigger fish to fry.
you might find some info in the osha webpage, but i think there may be more info in the federal motor vehicle safety standards (fmvss). info on the 121 braking system, standards for the composition of brake shoes, wheels and tire specs, etc, should be contained in their specs./ good luck wading through the muck of thousands of laws, rules, and regs.
remember that there may not be a "law" per sey, but rather a "standard" or regulation. either way, it applies. the exception may be private ownership of a non-commercial motor vehicle, also, the rules apply to the date of manufacture, so older vehicles are usually "grandfathered".
as to getting some idiot to change a true split rim on a 50 plus year old vehicle, good luck with that. not that they are not out there, but they are getting harder to find. most of the guys who were not killed, but rather lost fingers, hands, etc.. are long gone. maybe you will get lucky. in this part of the world the commercial tire shops refuse to fool with the rims, mostly i'm sure, to insurance. in the truck tire store where i buy my tires, there is a sign, with a picture, and they do a tremendous business with the farmers, and older rims. they refuse to change split rims, no argument. they refuse to change any multi piece rim that isn't inspected by the manager. any cracks, or excessive rust, and they point blank refuse to even remount the tire. they give you back the pieces along with the old tire, and tell you to have a nice day.
there are alternatives that weigh less, don't self destruct when they go flat, are cheaper, and any tire shop will change them, they run cooler, and last longer, but some guys insist on what they have for reasons known only to them.
in fact, due to the age of the truck, the rims might well be exempt from the current standard. older semi tractors (before 70 i think) didn't need to have front brakes---at all. those trucks are still legal on the highway, but, remember the highway was mostly all 55 mph for big trucks back then (interstate) and 50 on secondary roads and weight limits were much lower. your truck was probably made for 40-50 mph "good roads", and a much lower weight limit.
my calender says 2006.
