[Dave in MN]

I am in agreement with Brent. Proper rear cap alignment, bearing clearances and crankshaft end play will give you a leak free engine with the original oil slinger.

I, like Brent, also build engines. I have not found a good after market seal for the "A" crankshaft that is consistently trouble free.

If you insist on using something else that stock, I would suggest the Burtz seal. Caution: from my first hand experience, the rear slinger needs to be dead-on concentric with the rear main journal so that means it needs to be ground and polished while the crankshaft is at the crank grinder.
Good Day!