It should fit as long as your rim has the correct backspacing. A 15x9 or 15x9.5 would be about the max on a 69/70. Definately measure for clearances to see if modifications are needed. Sorry, not sure on Drag tires.
These are approx tire measurements:
Tread Width: 8.9"
Overall Width: 11.2"
Rim Size: 7.5" to 11.0"
I'm runnng BFG T/A 275/50/15's on 15x8" rims,but I had to use a narrower diff to get them under the guards, as the rims had only 3.75"backspace.Check Jay's chart.
The original diff had a bias of about 1/2" to the passenger side of the car,& that also helped to compound the clearance problem on that side.With another 3/4" of backspace the wheel/tyre combo would have fittted with the original diff.
On the street I am running 275-40-17's on a 17 x 9.5" wheel with 6" backspace. You should have no problem as long as you get the right backspacing. Good luck. As for slicks, I use a 28.0 x 9.0 x 15 and have no problems(15 x 8w/5" bs)
easy...i've put 275's on 15 by 7 rims on 69's before..
I currently have 295/50-15's on 15 by 8.5 (yes 8.5...centerline's sillyness) on the back of my 70 and had them on my 69 (don't remember backspacing; something around 4.5"...rubs in extreme situations but there's enough inside space available for more backspace and I could trim the wheel opening lip)
Mustangs of our years came with a zero offset rim, that means that if you have a 8 inch wide wheel you should have 4inches of back spacing. I have found that with a 275 on a correct rim is about the max. A bigger tire pinched on a smaller rim will fit. I run 265's with no problems.