Thank you Ted,  I guess I should preface this with saying that the Best Gasket seals did not work in this particular engine.  We found that the rope seal was impossible (for us) to put in the engine.  We were taking an engine to Tim McMaster for overhaul, and spoke to him.  He was not a fan of Best Gaskets and said that he exclusively uses the Chrysler seal on the engine he builds.  After hearing that, we ordered the Chrysler seal, trimmed the flaps and also trimmed the small extensions on the exterior of the seal and installed it.  Bingo!  We started the engine and ran it for 30 minutes with no leaks!   If the Best Gasket seal works for you, then use it,  After 3 failures and rewatching the video to make sure that everything was done according to the instructions, we are probably going to continue to use the Chrysler seal in the future.  Other's experiences may be different.

The crankshaft seal surface for the 292 and smaller Y engines is 2.625” while the crankshaft seal surface for the 312 engines is 2.750”.  Hence the need for two different neoprene seals for the Ford Y-Block.  The Fel-Pro seal is specifically for the 239/292 engines and will not work in the 312 engines.  Installing the 292 neoprene seal in the 312 engine simply redefines how bad a rear seal can leak in a 312.

My first experience with the Chrysler/Mopar seal was with a customers’ 312 engine many years ago that came into the shop and was leaking profusely from the rear seal.  That MoPar seal had been hand cut to fit the 312 and because of the rough cut, simply leaked.  Working on the rear main seal by simply removing the oil pan with the engine still in the chassis, the seal was replaced with a correctly trimmed Mopar neoprene seal and that fixed the leak.

Prior to Best Gasket coming out with a neoprene seal for the 312 engines, I was using the Chrysler seals that were modified and sold through FordCraft (Don Hyde) in Dallas.  Those worked well and I had no issues with using them.  It was common knowledge at the time that a number of 312’s did have irregularities in both the seal groove width and depths which would be problematic when installing a neoprene seal.  When those issues came to the forefront, rope seals were the fix for that.

When Best Gasket initially came out with a specific seal for the 312, it was orange, but those seals used a softer material and would fail in less than 500 miles.  BstGskt then switched to using the same black material that is used in the 239/256/272/292 seals at which point the wear problem with the earlier orange seals went away.  I have used the BstGskt seals for the 312’s with great success but I am aware of some 312’s having a wider/deeper seal groove which could end up being a problem with the seals not doing their job.  When I do find a problem with the rear main seals, more often than not the issue is with the side seals not sealing adequately at the seal retainer to block interface.  A dollop of RTV in the side seal groove before installing the side seals seems to fix that problem.

Here’s the link to a discussion a few years back where I went into detail on a rear main seal leakage issue that came into the shop.  Although this was a 292 engine, some of the same pitfalls you will have with a 312 engine was taking place here.
http://forums.y-blocksforever.com/FindPost145797.aspx

Lorena, Texas (South of Waco)

The 312 crank is larger in diameter

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!!!

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. 

I've used Chrysler seals for over 25 years

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 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. 

  

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 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.