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. 

  

Success after the 6th attempt.  We tried 3 of the Best Gasket seal kits and they all leaked,  we then tried to use the Sneaky Pete Chinese finger tool to pull through a rope seal, Two attempts and no luck there.  We lastly tried (at the suggestion of Tim McMaster) the Chrysler seal.  this needed the ears trimmed off of it and also the rubber at the underside of the seal also needed to be trimmed so that the seal will sit down in the retainers.  This finally worked.  we ran the car for 35 minutes on the rack once the seal was installed and NO LEAKS from the rear main.  The Chrysler seal was only $12 each, where as the Best Gasket one is $60.00.  The only problem is that the Chrysler seal does not come with the side seals, but we salvaged them from the rope seal kit.  This vehicle has been on or lift for 3 weeks while we tried everything.  Thank God it is fixed.  

I've used Chrysler seals for over 25 years

I would be interested in seeing what Ted has to say about the Chrysler seal. I have to question the use of any part that has to be modified for an engine for which it was not designed. In a 2009 post, Ted said that the Felpro, Best, ROI, and other seals are basically the same, but the side seals can be somewhat different. He specifically said that he has used the Felpro neoprene seal "with great success."

Could it be that replacing the seal with the engine installed is causing the problem? No doubt that it makes replacing the seal more difficult. 

Fel Pro does not make a neoprene seal for the 312,  They only make if for the 292-272-239 engines.  The main difference is the width of the channel the seal is wider on the 312.  The Fel Pro seal for the Chrysler works, as we ran the engine after installation and it did not leak.  It leaked like a sieve on all three tries with the Best Gasket seal.  I followed all the instructions and spoke to a couple of experts and it did not work.  I was told that about 10% will not seal with the neoprene seal due to machining tolerances.  I spoke with Tim Mc Master, "The Y Block Guy" and he uses the Chrysler seals in all of his rebuilds.  I am a believer now!  Maybe we are doing something wrong, but we watched the Thunderbird Garage video on the installation and we could not get the seals to work.  $180 in seals down the drain!!!