I *just* finished a rear main seal re-install today on my 512 so I sympathize with what a pain in the ass you're going through.

My rear main seal holder is a BCR one -- I run their whole main cap and girdle setup. The seal holder is a nice piece, but you still need to be careful. A guarantee of zero leaks it is not. In fact, you'll probably find the lip that hangs into the block and over main #5 will interfere with the nuts for the main cap if you're running studs. I didn't notice that when I built my motor last year, have had a leak since, and just finished getting around to fixing it today. In the following image you can see how I added clearance to the seal holder today:

Image of the modified holder

Other words of advice:
-Put the seal ends at 6 and 12 o'clock (I gleaned this from a post by Monte Smith and also RAMM)
-Apply aviation gasket sealant on the seal ends and onto the block where the seal holder sits. Don't use RTV -- it can cause adhesion issues or not let the seal holder sit down.
-If you're weird like me... I widen the grooves where the side wipers would go and don't use them. I let the seal holder naturally guide itself over the seal (with ends at 6 and 12) and to where it wants to. Then I do up the bolts and pay attention not let them shift the seal holder. After that I pump ultra black or the Right Stuff down the grooves where the side seals would normally go.

I hope at least some of that might be of help to you.