My 46 which is an Art Deco 1/2 ton the fenders mount on top of the R/B and there an aftermarket gasket that goes in there. If you want to be sure the fender is right there should also be fender mounting brackets on the firewall that keep the fender lined up to the firewall. If there aligned alright than the problem is with the running board. Possibly the R/B mounting brackets are bent but they should snug up to the cab below the door. Hope that helps and good luck. Bob T