I have a set of 200lb-in flex-a-forms in my 66. I haven't autocrossed with the car yet (next season as the car was being upgraded for most of this season) but the car is very predictable and the lateral movement is little to none. I also don't have wheel hop and don't have cal-tracs even with quite spirited, aggressive driving. If you can afford them and would like the weight savings, I'd recommend them.
The difference in ride was enormous, but to be honest I can't blame that all on the springs. I switched from worn out rubber and sagging 4 leafs to the flex-a-forms but at the same time added subframe connectors and poly "racing" shackles.
The ride now is firm but not harsh and the springs recover extremely quickly over bumps so that the car doesn't move up and down much when moving over rough roads. The rear end isn't as "live" feeling any more when one side hits a bump--I could almost feel as if I had an independent rear or at least a 3 or 4 link like the Fox bodies. When cornering on uneven pavement I used to skip sideways a foot or more as I bounced over the bumps. Now, over the same roads I don't have any of that "lane change" tendency. The stability is something I'm really enjoying and the car feels quite balanced now with the GR-2 shocks on both ends and the 480 springs up front.
As far as weight distribution, I don't notice any increase in understeer due to weight loss in the back. I'm not yet neutral or Corvette-like, but I'm not an oversteering pig, either. I still have a ways to go chassis and front
suspension-wise before I'm up to that Mustang II's handling that was referenced above.
One minor disappointment I had was that the ride height was higher than I wanted. I asked for the "road race" style springs, which meant that I remained at stock height (27" from ground to lower edge of wheel well top). But... that's 2" higher than I wanted to be. I did drop almost an inch by reversing the eyes, but still want another 1/2 -1". Long story short, I would have specified a 2" drop. The 1" drop (after reversing the eyes) is an inch too high for me.
As far as lowering blocks, I'm going to compromise ride heights and experiment with 1/2" lexan or acetal blocks. Chamfering (rounding) the edges and using the supplied plastic (acetal) 1/16" spacers should be safe and not damage the composites nor create much of a moment arm that could induce wheelhop.
Daniel
It started to get expensive and the cals negated part of the weight factor.
