Don't want to go down a similar rabbit hole with Andy
Our cut off date for Classic Clubmans was specifically to allow up to the Mallock Mk21 but exclude later cars with better suspension geometry, we didn't get into ground effects etc until later on.
Clubmans are technically defined as "Sports Racing Cars" and yes a natural development of the Lotus 7. You're right Andy they have run as single seaters, in fact a couple of Mallocks were specifically built as Formula Fords (and can still be see at the Gold Cup)
Interesting point about the engine's, maybe the answer is to allow cars that comply with the current Calssic Clubmans regs, which is all crossflows, K engines run in the modern Clubmans Cup formula but Vauxhalls are not used in the register championship.