Basically there are long and short depth caps of this type. Sounds like you want the long one. You'll often get boil over after a good fast run when you get back into stop go traffic as the heat soak expands the coolant a bit if the cap doesn't seal. It should only be a 4 lb one but if you can't find one, likely, a 7lb one will be OK. B20 uses a 10 lb one so I suspect that one won't give you any problems either if the rest of the system is in good condition.
Stay cool.