The body color white is the original gel coat Wrenn and as far as the pan to body gap I recall that mine just dropped onto the pan and must have had enough area as I sealed the pan before the frame was set and recall having enough contact area. . It was some 30 years ago when I did this and the cut and alignment of the donor chassis is critical before welding.
I also cut out the pans before I cut the chassis and then added the cut and modified pans. The floor pans were rusted out on my donor.
The person who can lead you in the right direction with the most experience as he is currently still building cars would be Alan Merklin (AKA Dr. Clock).
The building of these cars takes more time than the book says and to make it your own adds to that time. It can be challenging,frustrating and a lot off times it comes together the way you envisioned it.
Have fun and when in doubt just reach out.
Michael