Welcome to CHR, Mark. On a fiberglass body the floor is bonded to the body, and provides the bolting surfact to the frame. When you order a body from a supplier one of the questions is if you want it channeled and how much, as they make the body and floor separate, and then the good ones set the body and floor on your frame and bond everything together once it is all lined up. To channel it after the fact you are going to have to either 1) cut the whole floor out, drop the body down to the desired point and bond it back together, or 2) cut slots for the frame, drop the body and floor down to the desired point, and build up new floor sections accomodating the frame slots. Does that make sense? Fiberglass is very forgiving, but for this channeling after the fact you're going to want to incorporate some bracing for strength.

Your SBC is pretty short compared to a Ford, so you shouldn't have any problem with space. If you need to notch the firewall it will be an easy task with glass.

Post up some pictures when you get it, and as you go along making it yours.