The bypass oil filter needs a flow-restricting orifice in its supply fitting to prevent too much oil to flow through the filter, which would reduce the oil pressure. I believe that the orifice diameter is about 1/16". There could be no harm in simply disconnecting the bypass oil filter to see if that helps the oil pressure.
I do not recall, back when I worked on Stovebolts fifty years ago with bypass filters, if the canister would have oil in it when the cover was removed. As the outlet is higher than the return hole in the block, there is no reason why the oil should not all drain into the oil pan.
This question has come up before, and I am sure that are members more current experience with bypass filters. I have converted my 1954 261 from no filter to full flow filter.
