Build the box. It may be easier to lay it in place under the dash and attempt to install the cables if you can't get at them when the box is up under the dash. Feed the heater tubes through and stuff the box in place. Cake.
Thanks........that's one question I had........is if there is enough slack in the heater cables to install with box out or
does it have to be installed first?
There does seem to be some room to access top of heater box with the glove box insert out.
