It looks like that mold may make a body very similar to mine. I am using a Dave Koorey Street Rods body and it has the cowl area extended 6". You definitely need the room if you're running 3 pedals; it was a tight squeeze for mine even with the extra length.

As for thickness: I have cut / drilled holes in several places for various things and this body seems to be about 1/4" thick everywhere except the floor and trans tunnel which are even thicker. I'm pretty sure this body was made with a chop-gun except for the floor reinforcement...

I worked at Eckler's Corvette years ago and most of their stuff had 4 layers of mat, but I don't know what weight.