Matt and resin will make a bigger mess, but it's really the only right way to fill that big of a gap. The glass impregated filler is good, Evercoat has a fine strand out know with Kevlar added to it, but I think you would be much better off building the majority of fill with 'glass. The filler is good, but down the road with all the pounding and shaking of the road, and lots of good old heat from the sun that much filler would quite probably give you grief....