P6B radiators run with a gap at the top, the water level sits about 3 inches plus (75mm or more) below the fill level of the radiator cap it looks very disconcerting and a real trap for young players but nothing you need worry about, it's covered in the owners manual.
Quite a few members here have fitted an overflow tank un-pressurised and pressurised ones, a search here will probably show some good examples and how too's If its more than that you may have a leak somewhere.
There are heater hoses that run under the inlet manifold which are a bit hard to inspect and if too old could be a problem, I had one blow miles from anywhere up the Whangamoa
Several litres is a lot,, can you be more specific about the amount and which model P6 you have and check that it is not getting into the oil. Systems without an expansion tank will usually find there own coolant level where adding more will just get pushed out again but several litres is much more than I would expect, there is only 8 or 10 litres in the whole system
I presume the P5B is very similar, since ours likes to settle with the coolant about an inch below the filler cap. I wouldn't feel happy about using a 15 lb cap since the higher operating pressure might cause leaks to appear somewhere as well as putting a bit more stress on the radiator core.
To give better cooling we've also fitted an extra small radiator in front of the original, to provide separate cooling for the autotrans fluid, keeping it well away from any potential water ingress.
I replaced my rad cap with a new 15lb one and a new lower temp level thermostat. pressure then found a few small leak areas. The pipe under inlet manifold for example . DO check the small thin pipe from inlet manifold 'tower' back to rad often gets blocked as small diameter and WILL cause overheating and loss of coolant. Try and check water flow in rad across cooling fins by starting cold and revving with cap off...ought to see a slight flow of water slowly get faster as thermostat starts to open as it warms.
we might need to pressure test system if we get leaks with a new cap? any pain holes will pop pressure in system. Anti freeze tends to leave stains where it leaks from..so check for those and any loss under car. Mine often vented from the overflow pipe from rad. I later removed and cleaned rad and replaced all pipes . Still had a slight loss of fluid so added 'bars leaks' that problem now solved now running with waterless coolant ( Evans) too as it helps with cooling . hope we a have decent engines with no overheating issues .