Those front/rear attachments look pretty strong! I recommend that after you know your rear corner weights of the car, you then determine the poundage so to further determine the free arch and compressed arch. You should consider about a 1" remaining arch after weight of the rear is compressing for best performance handling (near flat).

I've used FLEX-A-FORM mono-fibergalss leafs since the late 1980s and never experienced any fatigue problems (cracking, etc). The fglass springs do, however, take about 2-weeks to settle to the permanent arch (as per my experience)... so, you could add some extra weight to the car during its initial resting time after you install them.

Mopar Mitch "Road racers and autocrossers go in deeper and come out harder!"... and rain never stops us from having fun with our cars... in fact, it makes us better drivers! Check out MOPAR ACTION MAGAZINE, August 2006 issue for feature article and specs on my autocross T/A!