After I assemble my 261, I had a tremendious oil leak after the maiden voyage. I also changed the seal from a rubber lip, installed by my assembler, to a rope. It was OK for awhile, but started again. I checked the valve cover gasket at the rear and resealed the cover with General Moters gasket coumpound, which is good stuff. I thought I had it. It leaked, I tightened the valve cover hold downs. It still leaked. Finally, I found it! This may be your problem - the pins on the end flange of the crank shaft that correctly line up the flywheel were under size and one was gone! The oil was going out that hole and and at the other two pins and gave all indications of being a rear main seal leak. If you don't know what it is, you would swear it was the rear main seal. As I said, this may not be it, but I have talked to several people who have had the same problem and when they checked, that was it! Good Luck!
