Distributor Swap Question

The OEM O-rings of the old days did a good job. I have heard but not experienced, that the newer aftermarket stuff does not always do the job.
I have experienced that the tin gaskets as found in the FelPro complete overhaul kits work very well. This gasket is not flat , but has a concentric rib on it. I try to use them whenever I can.
After you have proven where the oil is coming from, and if it is the dizzy, then you will have to check the fit of that O-ring.
More often, the oil-pressure sender leaks; either at the threads or right thru the bottom.