The flat style cup alone will not fix your leak. The cup will bottom out at the back of the bore and it will deform which will allow it to leak. I know this from experience. There would have to be a secondary piston behind the cup to act as a stop. This is the setup I have been running for for about...

Attempting to attach pics.

I'm not the OP, but I am also a new 69 van owner in Ok. Actually I bought the van from my grandmother and a 72 sedan from my mother. After setting for 20-30 years, they are both running. I just need to make them stop. With help from the book of reprints, I think I have all the parts I need.