Phil, you could put some webbing in the joint, or some fender bead to fill the gap. Fender bead wasn't used in the factory as as far as I know, however. These trucks weren't luxury vehicles and I don't believe the factory worried too much about fit.
My driver's side fender fit horribly when first installed. The very front of the cowl panel I welded in was too far outboard, so the last time I had the front clip off, I cut and rewelded the cowl to fix the fit. It was MUCH better after I did that.
Link showing the repair.
