I have done a half dozen big block builds (all differnt blocks/cranks), always used the stock retainer. Never had or heard of any leaks, some have several thousand miles on them--that's using a felpro seal too. I always put the block surface of a retainer just a touch on the belt sander to even it square---a file could do the trick too, use anerobic sealer between the retainer and block, and squeeze grey RTV in the side seals and I have used those little pipe cleaner thingys. I also offset the seal slightly with a dab of rtv on the ends. Works for me.

I will be looking into the superformance seal for the next build though, I believe it should be a good product from what I have heard.