i think most new stuff use a 16lb cap. and by new, i'm talking about the OEM 4-ear cap that came out in ???? model year.
make sure the cap you are using has the two gaskets, and the inner one fits and seals good on the bottom of the radiator neck.
also, be sure the overflow hose is tight on the radiator neck, and the catch can has a good vent to atmosphere.
sometimes it takes several cycles to expel all the air from the system, so don't let the overflow run dry.
you may even have to get the nose up in the air to coax the air in the system to rise to the top.
i squeeze the upper hose while filling. that seems to help burp it some.

I've typically run 16lb caps on most of the 60-70's vehicles unless they had an aluminum radiator. Some of the MFR's of them say 6-7lbs max. Likely due to non ribbed tanks

The purpose of the higher rating isn't to keep it from puking, well not directly. It raises the system pressure so you can run the coolant hotter without it boiling over. The temperature increase is the reason, emissions are lower with higher operating temperatures..

Just like engine oil I don't think it matters much. Use a high quality anti-freeze and distilled water. Don't mix brands, just pick one and stick with it. Don't use off brand stuff, pick something good and stay with it. Change it every 2 or 3 years.