Try a dry run without the end rubber and see how much of a gap you have. Sounds like a simple case of using a gasket thats too thick for the application. Small block Chevy gasket sets always include two different thicknesses of pan end gaskets for this reason, maybe you need to get a different one than what's included in the gasket set you have. If the gap isn't real excesive, you might be able to make your own buy laying in a good bead of silicone and letting it firm up, then add a little fresh stuff to the pan and then install. Let everything dry before running the engine.
