Most of the parts suppliers sell them now under part no. A6334 (Snyders) This is a bronze thrust face and labrinth type seal that replaces the original aluminum seal. Works great other than you have to have the engine out for overhaul as the rear babbit thrust face of the rear main must be cut or machined off flush with the block.
Any of the other type of seals offered that I have heard about, require machining of the crankshaft. (Not a plan in my book)