On my 73 Scamp I have 15x12's with a 4.5" Backspace. 315/60/15 Drag Radial
Face of Drum to Face of Drum the total width is 50.25" The wheel lip is trimmed on it in addition to the mini tub.
On my 67 Dart I have 15x10's with 5.5 Backspace. 325/50/15's Drag Radial
Rotor Hat to Rotor Hat the total width is 52.5". This one the factory wheel lip is un touched with the trim still around the wheel well.
For housing length you would subtract 5.5" Total. 1/4" for rotors and 2.5" per side for brake stand out.