Fiberglassing dash and interior panels

I'll be starting a project for a '73 Mustang hatch in the next week or so.  First off, they want the entire dash fiberglassed and painted, but besides adding tweeter pods, they want it to look stock.  Because the dash has some holes in it, I'm just going to lay a few layers of FG over the entire thing.  My first question is, part of it is heavily padded, and covered in vynil.  If I remove it, i will lose the whole look.  Can I fiberglass over this without it warping too much?

I'm also remaking the side panel in the hatch area to add a sub box.  I'm going to cut a piece of MDF to get the footprint I want, build a box behind it, and stretch the fleece up to the weatherstripping.  The problem is, the panel I'm remaking is a single piece that goes from the back seat (it's a 2 door) to the hatch.  what is the best way to cut this panel in half and attach the origional front to the new back?