I have always run the stock seal retainer. I recently worked on a freinds new 440 with a leak. He had bought a "bilet" retainer. It still leaked. I installed it to the block and measured it up. The bolts didn't even center it in the block register.It was off .100". I had him order a new seal from Greg at
superformanceproducts@yahoo.com
Installed it in the factory retainer, problem solved. I've never had a rear main leak on any of my small or big blocks.
Doug