I used a neoprene seal in mine and here is what I did:
polish the rear main seal contact surface really well on the crank (this is a must)
using a piece of wax paper or dollar bill determine how well the rear seal contacts your crank when it is installed...you want to feel resistance on top and bottom.
shim as needed...I had to shim mine about .006 on top and bottom seal...strips cut from high quality foil duct seal tape is very good to use.
if you have to shim more than .006 use a Dremel tool to remove some of the metal core (rod) from the two seal pieces
lubricate with motor assembly oil

Also...be really careful to check your side cover gasket (push rod cover). These are known for leaking and when they do (anyone can fail to get them sealed correctly), the leak mimics the rear main seal leak almost exactly.

Good luck!

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end