Closed Cooling System Questions

[dcdodge]

Have a few questions concerning the cooling system on my '56 GMC. First some background: engine is a 455 Pontiac coupled to a TH400 being cooled by a dedicated Dana unit. Radiator looks fairly new & has the Harrison ID's "R 211 55F 25505" on the top tank. Aftermarket heater is by Maradyne and a 16" SPAL electric fan cools the works. Found that the PO had shade-treed a closed system onto the truck with a 13# cap that was notched to fit the GMC bayonet neck. However this cap does not seal at all and easily spews out coolant during the hot weather. Planning to clean out the cooling system but wanted to get some feedback on:
1) maximum pressure for this radiator? Realize the OE specs were 4 or 7 psig but wondered if this radiator & heater core could survive the elevated levels of 13 - 15 psig?
2) Prefer to have a correct closed system but will need a source for the proper cap for this vintage radiator. Either rated @ 4, 7 or 13 psig. TIA!