Not to pick on any one individual but you read through these threads about painting frames (and there are many) and you'd think these guys are building something that has to last for a thousand years at the bottom of the ocean or something.
It's simply overkill and it's pervading just about every aspect of our society. If you don't toast your bread in a $300 toaster, you're not doing it right. If your car's glovebox doesn't have titanium roller bearings it's somehow inferior to one that does... and it just goes on and on and on.
It's an old truck frame. It's lasted 60 years with a cursory spritz of cheap paint. Why on earth would you feel compelled to treat it like it's going to dissolve overnight if you put a couple coats of (god forgive me) enamel on it?