68 barracuda tire size

A general rule of thumb is to keep the tread width within 1/2" of the rim width. I believe 14" BBP Rallyes came in 5-1/2" and 6" rim widths. The SBP optional rims (Formula S and Rallye) were 5-1/2". 225-70 X 14 would be the absolute limit for 5-1/2" rims. For many years, Ford Rangers used that size tire which is probably why they are still around.

If you truly have BBP all the way around, that opens it up for MANY more options than having SBP. If you are in love with Mopar Rallyes, you can get 15 X 7 in both small and big bolt patterns. (Summit Racing is one source) The standard back spacing for Mopar 15 x 7 wheels is 4-1/4" and those will fit without issues except......if you go too wide or too tall for the front tires, they will hit the wheel lip.

In my opinion, 255-60 x 15 on the rear of second gen Barracudas looks the best.