How do you determine what radiator cap you need?
 

A friend of mine built a 65 Mustang running an EFI swapped 351 Windsor. He has a 4 core stock replacement style radiator along with I believe a stock Lightning water pump. Hoses are metal tubes with rubber elbows on the ends. After driving it has a small leak from the lower hose and he can't figure out why. I am wondering if the system is building too much pressure as its running whatever rated cap that came with the radiator. How do you figure out what radiator cap is needed?

I was just wondering this myself! From my understanding higher pressure raises the boiling point of fluid so it can get hotter without steaming. I would imagine your freeze plugs and hoses would all be able to handle higher pressures, I wouldn't think that systems running 20 lb caps have different hoses and such than a 12 lb system.

Do the metal tubes have any sort of beading that the hose slides over or are they just plain elbows?

I haven't seen the metal tubing but I am assuming they are just flat tubes with no bead or anything. I told him to try the hairspray trick that people use on intercooler piping or maybe throw a bead of weld around the pipe. Maybe that's the ticket.