Not necessary to remove cam in order to replace valve stem seals. Question: just how are you trying to adjust the valve clearance? Valves are threaded, and adjuster is secured with a double nut locking provision, are you sure you have the lock nut backed off? Original question, yes you can replace cam without replacing head gasget, not necessary to remove head at all, and removing cam towers will not effect gasget seal.

Turbo 240