Thanks guys for the suggestions and help. I have tried the rubber gaskets available from the usual places, but they are thinner than some of my old inner tubes. It would take several to fill the gaps and at the same time this would separate the areas that currently fit tightly together. Still not sure just what I will do about the “fit”.
As far as fitting the entire fender. I started by installing the running boards, the main front brace, the inner fender and the front side splash pan all onto the truck loosly. Next I sat the fender on the running board and main front brace. The fender would sit there all by itself. Then I started with some 2 and 3 inch long 3/16” bolts through the various attachment holes, slowly pulling them tighter until I was able to get the correct size bolts into adjacent holes. The whole thing went a little easer than expected. The only problem is that the rear braces (just in front of the firewall) seem to be a ½ inch too tall.