No matter how careful I am or how quickly I try to shut off the gas, if I fill my tanks all the way up, then park the bronco in the sun, it warms up and burps quite a bit of gas out the gas caps unless I drive around and burn a bit off.

It's a 1970 and I don't see any evidence of charcoal filters or other venting systems.

And my garage always smells of gas.

'70 or '71 is when Ford switched from vented caps to non-vented caps and tank venting with the separater/condensor tank/charcoal canister. If you don't have the vent lines coming from the tanks you don't really have much choice unless you want to add some.

So is burping gas a well known Bronco feature?

I remember years ago a mechanic put sealed gas caps on it to fix the problem. I'd run for a while then the engine would just stall out. When I took the cap off there was a large pressure relief hiss, so I went back to vented.