I have been going through the same thing on my 512 (World M/P Block). Leaked on the dyno and I resealed it; leaked in the car and I pulled the motor to inspect it. There was a thread about World blocks being machined wrong for the seal but mine looked ok. I was about to get PO'd and then I seen that there were 2 deep grooves on the top inside of the seal reatainer; I pulled #5 cap and sure enough, the cap was hitting the seal retainer preventing it from seating all of the way. I have used these mancini billet side sealed o ring retainers on several of my builds and never had a problem. I looked at the retainer that came with the block and it is machined a lot more in order to clear the cap. This probably is not your situation but you might check and ensure the retainer is not hitting #5 cap.