Molding in EVO II style fender flares?

[pro60modman]

I have a EVO II body kit from raceonusa and the fender flares have to molded in to the body. They claim the kit is made of a material called "Hi-flex." I assume thats is a another term for fiberglass? Anyway I have done my share of fender flares but mosly off road trucks. I know I have to cut the fenders and weld the inner and outer fender back together but I'm not sure exactly what to use to join the 190e's steel body to these "hi-flex" fender flares.

Any suggestions? I like to do all my own work so please dont say body shop