Welcome to the forum! First of all, a Pantera is like an Italian suit. Each one fits differently based on various combinations of the seats and the steering wheel. I recommend "trying on" several Panteras before reaching any definite conclusions.
If you do go down the lowered floorpan route, it's not difficult from what I understand, and there should be no impact to your paint - unless you're careless with tools and bang things around.
You also have several options for lowered pans, so check with the various vendors and search this forum and the DeTomaso mail list...
One possibility is to remove the top layer of the inner floorpan (what you see under the carpet) and bolt the seats/tracks to the bottom of the existing pan. This means the exterior pan remains unchanged, but you get a bit more headroom.
Check with the vendors on the various kits/processes. I'm sure others on this list who have done this mod will chime in too.