My Mega block uses a stock part. No side seals, just RTV. They can push the retainer off to one side causing a leak. Make sure the bolt holes don't bind holding the retainer to one side. If they do open the holes up one drill size and recheck. Best to bolt down the retainer with no crank during the build to see exactly how it lines up. Make sure the bolt bosses aren't cracked from using a bolt that was too long. Also fill the retainer bolt heads with RTV. Not all pans completely cover the seal retainer bolt recess.
Doug