I have a 512 M/P World 440 cast iron block that we put on the dyno in March. Made excellent power and torque, but started to leak from the rear main. After the dyno sessions I pulled the pan and put new side and main seals in the retainer (Mancini's double sided o ring retainer) Installed in my 67 Coronet and it started to leak again; I pulled the motor and looked at the seal area in the block and it looked fine; pulled #5 main cap and then I seen the problem. The Mancini retainer was hitting #5 cap preventing it from seating (there were 2 gouge marks on the retainer where it was hitting the main cap). I looked at the retainer that came with the block and it is machined to clear the main cap. Lot of work to find this out; once again don't assume that all of the aftermarket parts will fit different blocks. Hopefully; I can have a leak free motor this time

I hope what you found is the culprit for your leak

Any time an aftermarket part(s) are used EVERYTHING needs to be mocked up and checked.

Here's a pic of main studs interfering with a stock seal retainer causing the same issue
Photo courtesy of a Moparts member