Well, back-spacing is the most critical point in fitting wheels and tires. Mainly, because if you mount the tires and wheels, the wheels can't be taken back if the size is incorrect. I have 8" Weld Draglites and I went with the 5" rear spacing. I have BFG 275/60/R15's. I think there might be a little more room but maybe not much more than that. Those Mickey's are a ton of meat for the street. I went with this beefy combination for the muscle car look. Not sure if that set-up your talking about will ultimatly work or not. Most wheel places can direct you on how to get an accurate indication and fit. I would try Summit or Jeg's and talk to a tech, they should get you all set up.
Rear wheel backspacing is the distance between the inboard edge of the wheel and the mounting surface of the wheel that bolts against the axle hub. It is measured by placing the wheel face down on a flat surface; lay a straight edge across the inboard edge of the wheel, and measure from the wheel mounting surface to the straight edge with a ruler.
On my 69, I require a 4.5" backspacing for my wheels...
Note this handy visual reference. On the Picture, "Back Spacing" is referred to as the "Rear Spacing"