IMO you will have trouble with 8 inch wide on a 66. I lot depends on your suspension and what tire you run.

 

Here is a link to Dodgestangs wheel chart for 65-66. http://www.dodgestang.com/Tire%20Fit...ide%2065-6.pdf

 

A 15x8 wheel will need 4.5" of backspacing.

 

A 15x8 wheel with the same backspacing (4.25) will stick out 1 inch further that a 15x7 wheel with 4.25 bs. If you wanted to put the centerline of the wheel in the same physical location you would need to use 4.75 backspacing. You can't use 4.75 backspacing on a 15 or a 16 inch wheel because you run into the upper control arm. The max BS you can use on a 15 and 16 inch wheel is 4.5 backspacing.

 

For the front 15x8 will be too wide since you will only be running a 215 or 225 tire. It will look goofy.

 

I'm running 15x7 front and back with 3.75" backspacing
front are 205/60R15, rear are 225/60R15
no rubbing on mine but I haven't done any lowering in the front. if I get new wheels they'll be 4.25" backspacing 15x7 as well

 

FWIW, I just swapped from 225/50-15 to 225/60-15 on 15x7 with 4.25" BS and a Global West F&R suspension (dropped a few inches) and it clears fine without rolled front fenders.

 

16 x 8 wheels
4.5 Back Spacing
TT2 rims with 225 50VR 16 tires all around
No rubbing

 

It don't know why people use 50 series tires on 15'' wheels, makes it look like a low rider or something with little doughnut tires, 60 series is muscle car!

 

Low riders FTW! I run 50 series on my 15s cause 60 is too tall and I don't like cars that sit up high. My 67 is now just 4-6" off the ground with 50s.