I have those exact arches on mine.. well sort of.
Mine were some cheap copies from a show. They wernt symetrical, they were wobbly and the gel coat was naff. But it didnt matter to me cause I wanted to blend them in.
Mine have been on the car and in use for about 18 months now, and only 1 has cracked at the front where I bumped into a high kurb.
I used proper fibreglass matting and tetrosyl resin to fibreglass them on. I stripped the area to bare metal and ran a soft sanding disk on a grinder (P24) over the gel coat.
I then screwed the arches on using the holes already in them. This basically placed the arch where I wanted it and kept it in place while I glassed it on. Although I did leave the screws in, then just cut them off from the otherside when I was done.
For the actually fibreglassing, I cut strips of fibreglass matting and applied them over the join, brushing on plenty of resin and making sure there were no big air bubbles. It only takes about half an hour to go off, but best to leave it a day or 2 to go really hard before you start working on it.
I then shaped what I had a little better quickly with a DA sander and 80 grit disks. This was uber itchy. Gloves, mask and eyewear are a good idea.
Then I started about filling the arches to the shape I wanted.
Each arch took me about 8 hours (I wasnt very good back then) to fill alone. Probably closer to 10 hours per arch with the fibreglassing and attachment.
As said, its highly likely they could crack, just make sure you use proper matting and not just fibreglass filler which is made for filling holes and such.
Anyway, heres a picture of how mine look.
http://www.dannytipp...acks-mini-2.jpgHope that helps.