I wouldn't worry about a half inch. You figure a longer, wider wheelbase contributes to more lateral holding power, albeit it minor, but what the heck.

I never thought about undercoating as a means to protect the spring. That is a good idea. It is semi-flexible, so that might be a good way to avoid the nicks that could lead to splintering without it cracking and flaking off. Sure would look better than wrapping them in duct tape.

The rate may be pretty decent. I don't recall if you have a rear sway bar or not, but if you don't you may not need to add one. If you do, you may want to experiment with disconnecting it. Not sure. I've never used fiberglass springs myself, but some of the oval track guys I've talked say you actually need to rate them higher than steel springs. I wouldn't know why, rate is rate, so maybe it is a old witch doctor's tale to confuse the competitors.