Since you said you swapped the base plate out for the 90 degree adapter I will assume the base isn't the problem.
1. It is common for the gasket to get stuck in the block around the filter adapter. A small chunk can go unnoticed. Remove the baseplate/adapter and be very very careful about making sure it's clean enough.
2. Check for oil trickling down the block from the end of the manifold/distributor/valve cover. Very common too.
3. Make sure you use new gaskets. Kits are available from the dealer for the 90 degree adapter. Don't expect silicone alone to take that kind of pressure without a good fiber gasket to grip to.
4. If you go back to the base plate use a 340/360/5.9 plate which has 6 to 8 holes or redrill the 4 hole 318 plate to have 8 holes.
5. Make sure there is a little space between the center of the plate and the block when no gaskets are installed. It provides crush on the gasket. If none exists a hammer can persuade it to have some.
No gasket under the center of the plate. Only around the edge.
Hope it helps