You might also see what condition your seal surface is in. In my case the surface looked like this:
http://i1154.photobucket.com/albums/p535/JonGoodman/ropesealcrankend.jpg

Because common sense told me that would eat the neoprene seal up in a hurry, I cleaned it very well and polished it only until I achieved a nice smooth finish with no craters:
http://s1154.photobucket.com/user/JonGoodman/media/Crank%20polished.jpg.html

When I put it in for a test fit, I shined a bright light on the back side of the crank and could see a sliver of light on one side no matter where in rotation the crank was. It was also obvious the seal I had was not a tight edge fit in the groove, so I used adhesive foil tape (about .003 thick) to make a shim to bring the seal up snug against the seal surface and on the back side of the seal to make the 2 pieces fit tightly in the grooves. The lower seal is set in place with some gasket sealer, but the top seal isn't.

We'll see how it works when I get the engine in and started.
Jon

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