The stock floor pan has plenty of stregth to support a removable transmission tunnel. The torsion bar/transmission crossmember shapes the floor for the tailshaft portion of the tranny. You could easily cut the tunnel and form a peice of .032 alluminum for the tunnel. Drill and dzus fasten the alluminum to the floor. You will need to leave a 2" lip next to the firewall as this will be the most difficult area to seal. You will difineately need some bead rolls of some kind to give the panel some rigidity.

If you are simply trying to obtain a way to better access the bell housing bolts. It would be better to mark and drill the floor and get some plugs for the holes. Electrical box 1" plugs work nice. Just an option.

Leon