Removed the heater box from my 1966 mustang yesterday and of course the plenum fell apart. I am getting a new one but need some advise on how to install.

Do I attach it to the heater box before installing the heater box?

Can the whole thing be put in together in a car with factory air?

Thanks.

 

I'd attach it to the heater box first and move the A/C unit (only a few bolts) a little if necessary to get it to fit in. Besides I think it'll be difficult to install the defroster duct tubes with the A/C in the car.