Part of this I believe is due to preparation of the tank and flanges. The choice of polyester resin appears to have been a good one from a structural standpoint although I find the fumes given off by these resins quite offensive. The appropriate mask should be worn unless you're working under a fume hood. Fill with flox any areas of poor fit so as to obtain maximum strength and best seal with fiberglass cloth strips. Front seam only.
Results look very good.
Filler neck and vent tube prepared for floxing into position. Locate engine mount on firewall.