The radiator on a 408 has an in built expansion tank. When you look through the filler hole you will see a second hole about 5 " below and you only fill with coolant to about 1/2"below the second hole. Sounds like you have been over filling the system.
The manuals are available from Bristol cars and the Bristol owners club.
I have only heard of three of these engines blowing a core plug even at 45 years old. Heard of more core plug problems with 411 's. I wouldn't question the rad cap apart from if it's working ! Try a new one.
Greg
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