My son and I are working on a 1957 Thunderbird that has a leak in the rear main seal.  The car has a 312 and has a rear main leak. We removed the former rear main and found that the last mechanic used a Fel Pro seal,  the seal was put in without the slight offset of the seams on the seals.  They also did not put the two rubber seals on the aluminum lower cap, instead they just filled the void with silicone sealer.
First, I am told that the 312 rear main seal is different from the 292-272-239 engines, but several sellers showing the same seal fitting the 312 also.  In any case, we purchased the Best gasket rear mail seal kit and rotated it about 1/2" per instructions, from the lower retraining seal retainer, installed the rubber side seals per the instructions and used a small amount of RTV on the parts and reassembled.  Unfortunately, that seal also leaked.
The seal leaked so we removed the oil pan and pulled the seal.  We found that there was a kink in the end of one seal about 1/4 to 1/2 inch on one of the seals.  We also found that instead of the original 1/2" offset, the seal had moved about 1" from it's original position.  We contacted Best Gaskets but the owner was on vacation.  The person that answered the phone gave us the phone number for "Bob" who is in Wisconsin.  We called Bob and he said that the offset should only be 1/8" and that the 1/2" in the instructions was too much and caused the seal end to buckle.  
We are going to purchase another seal and have only 1/8" offset this time, but is there any other things we need to look for?

Yes, the 312 seal is different.

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I read the information from the older posts that Klutz provided.  I also ran the part number from the Fel Pro seal that was in the car and it comes back to a Chrysler rear main seal.  I remember years ago someone saying that the Chrysler seal could be modified to fit, but it obviously does not come with the side seals that were missing from the original install.

I use Chrysler seals, and have not had a problem, you may want to check your rear main bearing, a worn bearing will cause any seal to leak.

I have successfully used "The Right Stuff" in place of the Side Seals. Apply a thin Coat on the Parting Surface and fill the Sides with it. It will create an impervious seal between the Block and Seal Retainer. I did mine 5 Years ago on My 292 and when I took it apart recently I had to cut the Sealant apart to get the Retainer out.
.I just finished rebuilding it and used the same method to seal the Rear Main Seal. Just My experience with an alternative fix.