My "Opinion" is that any more than 4" of backspacing, total, will put the tires too close to the frame in the rear and too close to the stops and linkage in the front.
I did notice on my 80's frame the stops on the front, Left and right, were worn pretty badly. I guess the last owner's) made a lot of hard turns. I welded on an additional bead all along the stop to give it back the 1/4" it needed and looked like was originally there. Without that extra 1/4" my tires "sidewalls" would have rubbed on tight corners.
I have P255-50ZR16 on my 5th gen Elky and they don't rub. You're are narrower than mine are.
My wheels have 4" of backspacing and are 16 X 8" (3.75" backspacing with a .25" offset for a total of 4")
I am no expert here so like I said its my "Opinion" that 4" is the most.
The 73 has a larger body/frame, and should have an easier time fitting larger tires. Also, since you're using different rims, the backspacing could be an issue. Lastly, you're using a 60 series tire on the 78, and the sidewall "bulge" is a little more pronounced than on the 55 series. The tread width may be less, but the width of the tire at its widest point is the critical measurement.