any tire/wheel advantage to upgrade axles

Yup, improvement on all 3.

You can find 15", and even some 17", rims for SBP, but your choices are very limited. Tire selection for 14" rims is a joke, 15" isn't much better, but it is better.

SBP brakes are 10x1.75", going to BBP will let you run the 10x2.5" brakes that were on pretty much every other Mopar until the early 90's. And, if you really feel the need you can find a set of 11x2.5" brakes that came on the B and C bodies. Or get a rear disk kit, as those won't work with SBP either.

And of course the axles will be stronger as well.

I'd talk to Cass though (Dr. Diff), as he sells the BBP axles and can make sure they're the right length and offset for what you want to do. Lengths are slightly different for the Sure Grip axles, and the offsets are different for 10x2.5" brakes, so you'll want to make sure you get the right length and offset for what you want to do. Some of the new axles can only be used with green bearings too, so if you want to run the old style tapered bearings (better!) you'll want to make sure your new axles will accomodate them.

And don't forget you can upgrade your front brakes as well, not sure if you've got drums or disks, but if you've got SBP drums you can go to BBP disks without needing two different bolt patterns on the car. And that will really improve your brake options.