You really need to measure accurately and add for tire bulge when trying to determine backspace...
On my 70 I think I've heard others say the wheel well opening is larger than a 69.
I have 275/60/17's on the back of mine and it is stuffed...I also had to slide the rear end back with a spacer in front of the spring perch for the tires to clear the front bottom edge of the 1/4 panel. These tires I have measure 30" tall and have a 11 1/2" section width....
They are on 17 x 9 1/2 in rims with 5 1/2" of backspacing.
After the first year I ended up using a 1/4" wheel spacer behind them. They only ever rubbed on angled odd entrances and exits...
Using a 5" backspace would be conservative towards the spring side...and as mentioned above tire bulge and the overall length of your rear will change everything.