Tire Size 68 Barracuda

@72bluNblu is pretty much bang on with the 15” recommend(s)(and of course his 17&18" recommends), I’d add that after going through this on my last three A bodies that each of them can easily be different by+/- a 1/2 inch or so on it’s ‘available’ clearances Due to factory assembly inconsistencies.

On the rear- 275/60’s with an 8" - 8.5” wheel requires bang on backspacing at around 4.5 MAYBE EVEN 4.75” (up to 4 7/8” may work - this was a common Centerline backspacing) and even then you had better hope the diff is perfectly centred on the factory welded perches as it can get really close to the LEAF SPRING when you factor in sidewall bulge. Another sensitive area on the rear is the BOTTOM FRONT portion of the the REAR WHEEL OPENING. I’ve had some with basically no clearance and others with about 1/2". Some people shim the front spring mount to move the diff back a little to make the clearance more here. If I shimmed (which I haven’t) I’d use a solid piece of steel, don’t just build it up with washers. The last area of concern is the WHEEL OPENING LIP and the inside wheel house, kind of self explanatory here, 275’s in an 8” should clear the lip but it’s close. 9 and 10” wheels makes fitment even more difficult. On each case for me I never ended up using 275’s as I just couldn’t get a happy amount of clearance in the areas mentioned so opted for 27” x 10” rears (P255/60R 15’s)

On the front- I have found that about the most I can get in is a 26” tall tire. On this car a P215/60R15 was used and I have about, Maximum 3/8“ one one side and maybe a tad more on the other side clearance on the BOTTOM FRONT WHEEL OPENING and also at the REAR OPENING TOO, also where the fender bolts into the car about 8 inches up from the bottom Opening
Wheel width and backspacing is KEY on the front. You need the perfect formula to get the tallest tire you can in here. Which also can dictate the max size tire too. 235/60’s we’re definitely a NO go- too wide…..the height of a 225/60 should be okay but the extra width in the 225/60 vs a 215/65 can make it real close again too - it’s a bit of a juggling act between these two sizes and for me I preferred the height of the narrower taller 215/65 to achieve a 27” rear and 26” front staggered effect. 215/60’s fit with ease but looked too dinky…

for the record my rims dimensions on THIS car are:

15x8 rear with 4.25" back spacing and 4.75" outward or front spacing so to speak.
15x6 front with 3.75" back spacing and 3.625" outward or front spacing so to speak.

you’ll notice that if the rear and front spacing values are added together it equals an inch more than the rim width. When measuring back and front spacing you include the the lip of the rim into the value.

tires are:

P255/60R15 rear
P215/65R15 front

you could likely use a 7" wide rim on the front but would HAVE TO increase the amount of rear backspacing in order to make them work. The only issue here when you go to much rear spacing is inner fender well rub and potentially control arm interference etc.

good luck with your choices! Post a picture when done.