Cragar rims back spacing and rear tire width

[Geos 72]

Hello to everyone, sooner or later I want to put a set of 15x10 Cragar rims on the rear of my 72 Chevy Cheyenne C10 w/ a set of tires to fit the width of the rims. I don't want the tires to stick out past the wheel wells. I wonder if anyone would know what back spacing would the rims have to be and what width the tires would I need to have it all tucked under the box. I hope I'm explaining that right. Any help would be appreciated. Thank you, George