You wouldn't have to plug the distributor hole or the road draft tube. They should be isolated from the intake ports unless the intake gasket seal to the head is bad, or bad intake valve to seat sealing.
You might want to try removing the distributor and use a piece of hose from the dizzy hole to your ear to listen for leakage after putting compressed air into the carb flange as I described earlier.
There will some leakage past the stem seals. It shouldn't be that much if the clearance between the stems and guides are in spec, and the seals are good.
If you use the same hose or smaller one to get it closer to the seals and hear air you may have found your problem.
I thought the Viton seals were black, but I could be wrong?