Just my observation.....my 54 truck, 1960 235, 180 thermostat with a 7 lb cap and coolant just covering the core tubes has never needed coolant added and runs consistantly at the 180 thermostat temp. With the correct cap and plenty of room on top of the core for expansion the radiator should never boil over.
Adding a higher pressure cap with provisions to dump into a coolant recovery cannister and filling the radiator completely could easily put undue pressure on the radiator and also the heater core.
If your cooling system is in good shape and your fan blade located high on the radiator you shouldn't have cooling issues. I genuinely think that you should be able to run the 0 or 4 lb cap with plenty of room above the core for expansion and eliminate the need for an expansion tank. Hope this helps.
Dave
