fusor makes a product that bonds fiberglass to metal,Ive used it on bodykits before.....Id buy up some of that,take your fenders off,prep the area,apply the adhesive,bolt your fenders back on and fiberglass the edges so the seams are gone....but thats my method,im sure many ppl do it many different ways
Fusor em on as well as bolt em then let it dry, grind it good cuttin all the fusor off that u can see then use a good SMC/kitty hair filler, then finish out with 2 part flowable putty.