Chris, It's 5 1/2- 5 5/8" for the 1970. Every car is different as it is really pushing the limits of the wheel wells and it isn't unusual for the car to have more room on one side than the other. 69's need more B.S. than 1970-2 as a rule. Not sure. Maybe other guys will weigh in.
1/4" offset on the rear (5.75" BS on a 10" wheel) is perfectly fine on the 70, it's really close to the factory number. Not sure about the earlier ones tho...