Removal of A/C-heater box?

Sure. You might consider keeping the vent workable. I think I saw a thread on here where someone mounted the wrong side vent box on the pass side. I'm going to mount a cut - down Mustang vent in my left side, to provide more room

Here it is mocked up. The old 'Stang boxes have a cable operated butterfly

http://i60.tinypic.com/a2ufdk.jpg

This is what a 'stang vent starts out as

http://4.bp.blogspot.com/-GQEnAoPJ83E/UOb37mNOVbI/AAAAAAAAy04/ChnM-tq2U_s/s1600/IMG_8386.JPG

Just happened to find, here's one thread on "heater delete" vent box

Heater Delete VENT?