J Robinson.---I know a million things. About 300,000 of them, I learned from practical, hands on experience, building my own cars over the last 41 years. The rest I learned by reading a lot and talking to people I considered smarter or more experienced than me (they've grown fewer as I've grown older) The bit about not letting interior flooring or firewall or bulkheads come in direct contact with the fiberglass body shell falls into the latter category. Since it seemed to be a pretty well established fact in "fiberglass expert" circles, I have never taken the chance of doing it any other way. Deep down, I believe that it has to do with the different expansion/contraction rates of fiberglass versus plywood.---if you have a peice of plywood 3/4" wide touching the fiberglass body, then it is a "point contact" type of thing, and will leave a "shadow" on the finished body. If you leave a gap, and tie the plywood to the body with a "flange" of fiberglass about 3 to 4" wide, then the loading is spread out over a much greater surface area, and won't leave a shadow.----I think thats it, anyways.