Steve: Most, if not all, of the body manufacturers will tell you how much the pan needs to be shortened to match their bodies. I worked with a CMC body and their manual not only tells you the dimensions needed when finished, but how to do the cuts, shortening and welding to complete the job. Having said that, I also used an old reference article from Kit Car magazine to help me along. The CMC body requires a pan shortened 11.81 inches. Not sure what the others need (help, someone??) but I'm sure someone here can help.
I'm sure that JPS, VS and IM have the same types of assembly manuals for their kits and can make similar recommendations.
I would suggest that you decide on a body manufacturer, decide on which body style you prefer, and THEN you'll know how much you'll have to shorten your pan (and also which year(s) they recommend for a donor chassis). It's all pretty straight-forward and many of us on this forum can help as you go along.
Good Luck!
Gordon
The Speedster guy from Rhode Island