Need more tire...

Many people say the best tire that you can tuck on a 65-66 is about limited to a 245 sidewall. I gained an inch by minitubbing the very front of the wheelwells, but that would barely tuck a 275 tire. For a 295-300 to fit under the quarters, you will need to minitub the car. There is no way around that

I run a 275/40/17 but I dont see anyway of going any larger than that unless you want them sticking outside the fenders in the rear. They are mounted on 17x9.5" wheels, and I cant recall the rim offset. They do pretty good for not rubbing, but I do have a fairly stiff rear suspension too and my passengers are usually just my kids.

With those 275's and the offset I run, on the inside they are probably between 1/4" - 1/2" from my leaf springs, passenger side is actually a tad closer but I get no rubbing there since the sidewall is so short

You could do a leafspring relocation kit ( mopar kit ) it works and a mini tub. There are guys running a 28 x 10.5 tire with this set up.
ken