68 Barracuda. How Big Can I Go? Rim and Tire?

Sure did!
I installed L50-15s on adapters, on that 7.25, in 1976. It was a 6-cylinder, 1969 Barracuda, lol.
The factory tubs will easily accept 295/50-15s on 10s; but you'll need either a narrowed rear end, or a custom offset wheel ..... or Air-shocks..... which is what my car had... Hey I was 23 at the time and nobody told me it was uncool.