I had a 446 i built on the dyno several years ago..... The rear seal leaked so bad I thought I must have put 1/2 the seal in backwards.
It just ran right out....... But it started doing it right off the bat.

The seal was installed correctly, and ultimately it ended up being a distorted original seal cap that was the problem.
The oil was actually leaking around the seal in the cap, not "past" the sealing lip.

As Cab said, make sure you know where it's coming from before tearing anything apart.

Do you know if your crank has the grooves in the seal surface or not?

I have seen it where the grooves are pretty aggressive and just wear the lip right off the seal.

I also had a customer built stroker on the dyno once that started leaking really bad shortly after we got the motor running.
On that one, it ended up being I little chunk of the lip had gotten torn off the seal and that just created a little hole for the oil to poor out.