Biggest tire in 64 barracuda stock configuration

IF you are able to move the rear leaf in but stock wheel well you can run a 275 with the proper rim. My rims are 17x9 with offset +24mm works out to be 5.95 back space. The spring has to be moved over a full inch to make room for the tires.
I'm running 225's up front but I have converted to the large bolt pattern all round as well.

What are your plans for your 64 Barracuda? If you are converting to disc brakes or 4.5 bc? I would do the conversion first then choose your rims with the correct back space.

Good luck.