im molding mine on as well and i thought about this issue but the solution i found has been a rather expensive but great solution. for the adhesion of the fender to the steel i have used 3m panel bond 8115 which has a higher tensel strength then welding. only down side is its like 50 a tube or so and i am on my 6 tube or so for my hole kit. check my build thread out for pics. also to prevent the flexing from the widebody with the space under neith im useing 3m window weld to fill in the gaps underneth great adhersion and also when cured is very durable and prevents the fiberglass from flexing which is a big no no with molding. then for the filler to do the molding evercoat came out with this new filler called fibertech which is the strongest filler on the market being a kevlar reinforced filler. its kinda expensive at $60 a half gallon. between the steps that im takeing the fiberglass wont flex and crack let alone the body filler. as well as the fiberglass would literly break before the panel bond weld would break. panel bond is what they use now adays when you get into a crash and have to have a panel of the car replaced. the only thing that can get it off is laquor thinnner when its still workable and then opean flame when cured. so as long as you know how to use the products and how to apply them fears of molding kits in is a thing of the past.

my steps were......cut two inches out of the outer quarter leaving the inner quarter panel there. then i straightend the inner out to wrap the new outer and slit it to roll correctly, i welded it all together to keep body stregth and seam sealed all the gaps which were very few. after that i sprayed gatorguard bedliner up to where the flares bolt to keep the elements from damaging or rusting the new modifications. next i ground the body where the flare touched and glued with 3m two part epoxy. i riveted the flares while the epoxy was wet so i got great squeeze and then smeared the rest with a puddy spreader. after the expoxy dried for a couple of days i ground it smooth and applied fiberglass mat to eliminate straight line cracking. next applied the kitty hair fiberglass and then regrind. last is body filler for the finish look. after i primed the car and blocked three times. then paint. i also built the lower to be part of the side skirt . i like it removable not fully molded, and eventually thr car will have damage to the sideskirt and i dont want to paint half of the car or blend. i can just pull it off and repair.