Should I leave the heater box out?

I took mine out too. You will have to make a block off for the fresh air hole from the cowl.Or make something up to allow fresh air when wanted. "IF" you want you can install a electric "rear" window defroster under the dash and up to the defroster vents.Some times it will be good to have. I did this with a 72 GTX that happened to have a rear defroster,but you can get them from up sacle auto at the wrecking yards.