acceptable bs on a 18x8 mustang wheel...66 cuda

If you have four wheel disc brakes and large bolt pattern. Your car is not stock, there is no one here that can say for sure what backspacing you're going to need. Best bet is take some measurements. Probable going to be in the 5" range for backspacing if you move the rear springs in a little.

Edit
Just noticed the 70 cuda rear, don't know how much wider those are but probably a lot. Probably going to need a lot more bs than 5" on rear.