Pre '66 trucks, suitable for 4K+ hauling?

[Detroit80]

Wasn't really sure where to post this, as I don't yet have a particular truck I'm even looking at yet, and I didn't want to post the same thing in 5 different subsections.

I have a '74 Amerigo slide in truck camper that I need to haul, along with a flatbed trailer that loaded will likely max around 7500lbs (more often than not, it will be around 5000lbs between trailer weight and load, maybe less if I buy a smaller trailer). I've never had the camper on a scale, but from what others are saying online, it will be 3500-4000lbs ready to go camping. My current truck is an '05 short bed 1/2 ton, so that's not anywhere even remotely close to being capable at this amount of weight.

Now, I know that a newer truck will be magnitudes of order better at this, but I'm just flat not willing to drop $15K-$20K plus insurance costs, or even more a lot of times, on a truck that realistically will be used a max of a couple dozen times a year. Plus, I just really like the old iron, and am willing to accept the sacrifice in creature comforts and ride quality to get there.

What I'm looking at are 1966 and older 1 ton, dually trucks, and primarily wondering if the frames on these trucks are up to handling 4000lbs over the axle, plus another few thousand pounds of trailer weight behind them? I'm looking at '66 and older, so that I won't have to deal with emissions requirements here in Arizona on engine swaps, as I also have a '96 454/4L80E ready to drop into something, and a D60/D70 axle combo, as well as a D60/14bolt combo sitting here. So I know I'm good on drivetrain handling the weight, I just don't know if it's a good idea to stack 5,000lbs of weight on the older frames?