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We used to do this all the time. We used either a home made cylinder pressure rig consisting of a gutted spark plug with a coupling to connect it to a compressor airline or you can use a flexible rubber hose adapter for a compression test gauge and connect an air lint to it. Remove both rockers from the cylinder to be serviced, then apply 80psi or better air pressure via the hose rig thru the spark plug hole. Now you should be able to use a valve spring compressor safely without worry of dropping a valve because air pressure will hold the valve closed. Remove the spring keepers, retainer and springs, change the valve seal and reassemble spring parts and rocker arm. Move on to the next cylinder and repeat. You are correct to assume 45yr old valve stem seals are suspect if plugs are fowling. Careful inspection may reveal hardened seals and some may be shattered rendering them useless. Over oiling can be a problem for sure but a good set of replacement valve seals should help.