Even if it was posible to remove the half box ( which I think it is ) it makes no sense and would be harder try to keep just the other half of the box attached to firewall. Better to remove the full box and dissasemble the box on bench.

Is easier than it looks... just need to dissasemble the glove box lid and liner to get more room, and remove nuts from engine bay side of firewall, aside the hoses of course. Dunno if the heater box can be pulled out with blower still attached, but on 71/74 B bodies the A/C box is posible.