I have found that sender bulbs on modern replacement gauges (cheap offshore - planned to be replaced later) don't seem to have the same seat dimensions as older sender bulbs so they leak. They are close enough that they look like they should work, but they don't quite do the job.
Some later heads had larger holes set up for the electric sender (plain pipe thread)and need a bushing for the mechanical bulb gauges with the taperd seat.
My main experience on this is with fitting Canadian Pontiac 261 engine into a WW2 Canadian Army Chev that originally had a 216. I don't know how well my knowledge fits your situation.