The rear seal caps or retainers for the 312 blocks are now fewer than the blocks out there that need them.  At this point anyone that needs a rear seal retainer for their 312 will be more than happy with a fully machined replacement assuming it holds a seal that doesn’t leak.  Here’s the picture of a rear seal retainer off of a 312 I recently pulled apart that is cracked which will need a replacement.  RTV was gobbed on pretty heavily around the crack so it was obviously leaking.  Add this to the list of things to look for when the Y you're working on is leaking oil somewhere at the back.
 

Lorena, Texas (South of Waco)

Looks like it was machined with a deer antler.

As the paragraph indicates, careful installation is key,  I really doubt that the billet seal retainer will work any better than factory and the seal is commonly available.  Ted has instructions for free on his website, http://www.eatonbalancing.com/blog/2008/09/10/neoprene-rear-seal-installation-for-the-y-and-others/

http://www.ebay.com/itm/1956-62-Ford-Y-Block-312-Neoprene-Rear-Main-Seal-Kit-with-new-Billit-Retainer-/261803079510?pt=Motors_Car_Truck_Parts_Accessories&hash=item3cf4adf756&vxp=mtr



Lawrenceville, GA

See Ebay item # 261803079510. Has anyone tried this and if so, how well does it work? Kind of spendy at $230 but if it will keep the rear seal from leaking, it might be worth it.

When Injustice Becomes Law, Resistance Becomes Duty