I would recommend you try to buy premade bolt-on flares, even if they are for a different body.

I have made them by wrapping a thick piece of foam rubber around the tire, with the edge sticking out beyond the fender. Cover with wax paper, grind the body, then lay fiberglass from the body, out onto the rubber, until it's at least 1/8 inch thick. Trim to shape, bondo, sand, paint.