Hey Paul...
I had the same kind of leak with my 408W stroker in a 1974 (rope seal) block!!
I pulled the pan and replaced the two piece neoprene replacement seal 4 times with no luck and even pulled the engine to try a ROPE SEAL.....BAD IDEA!!!
Came to discover that in older blocks the rope groove was not very precise and some blocks wouldn't squeeze the two piece seal quite enough to seal on the crank!!
Found a sleeve kit (Fel-Pro "Ready Sleeve") and installed it on the rear seal crank journal and...VOILA!! NO MORE LEAK!!!
BTW, you can install it without pulling the engine!!
It increases the journal diameter by .009" and if you can find one for your crank it will GUARANTEE that you have no rear seal leaks!!
Good Luck,
Jack