On my 69 road runner, I drilled out the hole where the air grabber cable went. I ran the MSD box wires through a grommet on that hole and mounted box under the dash. There are two dimples on the fire wall just below the cowl panel seam on the passenger side on e-bodies (at least 70 and 71). You could drill out one of those holes with a step drill. On the inside of the car, it should come out just above the heater box. Unless you are already running a shaker then one of the dimples will be already drilled and used for shaker cable.