Suspension upgrades for my 65 Dart

So what rim size has the most options for performance tires currently? 18? 19? 20+?

Currently? Probably 19's and 20's if you factor in what will actually fit for tire height on these cars.

It really depends on the size though. If you stay in the 245 wide range there's quite a few options in 17" and 18". Like if you look at a 245/45/17 on TireRack there's 59 tires in the "performance" category and 91 overall. If you look at a 275 though all you get for tires that are about the right height is a 275/40/17, which isn't that tall (25.6"), and there's only 33 total available and 24 in the "performance" category. A 275/45/17 would be great, but no one makes them. A 285/40/17? There's 2. 295's? Nope. 18's go the same way, if you look at a 245/40/18 there's 148 tires and 103 in the performance category. Go to 275/35/18 and there's 63 and 53 in performance, go to 295/35/18 and there's 11. Yeah, 11. Not a lot of selection, and it drops off worse as you go wider from there. In 19's at 295/30/19 there's 34 tires.

So it depends on what you're doing, I just know that running staggered 275/35/18's and 295/35/18's I'm down to 2 tire choices at TireRack. Now TireRack doesn't show every tire on the market, but you get a pretty good idea. My next time around I may try to run square at 285/35/18, gets me back up to like 20 something choices.

Yep I’m running sbp and plan to keep it that way for a while. Thanks for help everyone.

Nothing wrong with that! The chassis stiffening will still help. And yeah, not many of us have unlimited budgets to do everything at once so doing the chassis work now will give you a great platform to keep building from in the future. Or not, if you're happy with the results! :D