15x8 wheel with 4.25 bs "biggest tire option without rub"

For a Barracuda with an A-body width 8 3/4 and stock spring locations your best bet for 275/60/15’s would be 15x8 with a 4.5” backspace for BBP axles, or 4.25” of backspace for SBP axles.

275’s are the limit for most Barracuda’s if you want the wheel to actually fit inside the wheel wells with a stock or lower ride height. Larger is only possible with a raised rear ride height, or offset springs.