The cap needs to have a double seal, one at the spring tip area and one at the flange area. This cap can be used for open or closed systems. The one w only one rubber seal can only be used on open systems. Pretty much all 74 mopar caps had a dual rubber seal and will fit most all 60's style radiators. The bottle will need to have 2 hoses, one that allows for expansion fluid to flow into to bottle from radiator fill neck, and that line also needs to go down into the bottle to the bottom so that as the radiator cools, it can suck fluid back in. The other hose is at the top and is for overflow in case the bottle gets too full. This type of system, you will fill radiator to the top, and just a small amount in bottle. Unfortunately I cannot recommend a bottle.
Is there a particular reason you are wanting to go closed system? I have a 496 w a/c in AZ w 125° summer temps. Open system. I fill 1" below bottom of filler neck. Have never, even on rare idling for 20 minutes w a/c on and engine hitting 225°, had it puke out or boil over.
