If the car is a non a/c car there is no need to remove heater box to get dash out. You may be able to get it of an a/c car without removal as well but I am not certain. Just disconnect the defrost tubes and heater control cables. The dash will lift out over the heater box.

70 Road Runner 383-4 4 speed FJ5 & black guts
70 Charger R/T 440-4 4 speed FJ5 & white guts