Do you have the thicker flanged aftermarket or cut down 5 on 4.5 axles?
Or are you running small bolt pattern or re-drilled small bolt axles?
I know with re-drilled small bolt axle a 245/50/15 with 4.5 backspacing, the tire will be about 1/2" from the leaf springs.
IIRC, the thicker axle flange move the rim out 1/8"? Or is it 1/4".
And your 265 wide tire is wider than a 245.
My guess is that you are going to need to run a 3/8 to 1/2" spacer to keep the tire from rubbing the leaf spring.
After that, start considering the space between the tire and the fender lip.
On a 68 Dart, I have 245/60/15 with 1/2" between tire and leaf spring. And it slightly rubs the fender lip. The rear bushing are bad and the leaf springs are shot.
I don't know if the 70 Dart has more rear fender lip clearance. But I doubt it.
IMHO, it's going to be tough. It would be very helpful if the rear fender lips were bent in (lipped). Do you have aftermarket quarters on your car? Most of those have a wider than stock lip. That will really screw things up.