Someone like Jay (Hotrodpaint) and others will have a more accurate answer, but to get you started, the old fashioned way was to weld the fenders in place and then lead the seam in to fill that gap. Lead is sort of old school now, and modern fillers like Rage and others are far superior for most repairs. However, none of them like to be very thick, or they will crack out. If it were me, I would weld them on and then use as thin of a coat of quality plastic filler as possible to smooth it out.

We'll see what the experts say though.

Don