rear main seal

[oldgasser]

 Could I get some information on the correct rear main seal that seems to be working for the 312 Wblock.
I have just finished installing a neoprene seal I believe I did everything right even had a engine rebuilder come over to my place & make sure that it went in right, anyway on its first run after it got hot she was leaking like a tap. The name of the seal was by Bests Gaskets out of California The 312 is in  my 57 Rachero.
Anyone have any suggestions?
Thank you for your time.
Ron.