[JLMounce]

At the power level your engine likely makes, you're likely beyond the true functional limits of the leaf springs on your car.

You can make up for it with a drag type tire, as you previously did, but almost no street tire is going to realistically cope here. I'm dealing with the same thing. I run an NT555 G2 and it's a nice tire that is comfortable, quiet and good looking. Not enough grip though to really handle the torque that is being put to the ground. Reducing the sidewall will also have a tendency to reduce grip when the tire is shocked, unlike the 15" tire you have which will allow a small amount of give to absorb the initial shock of a throttle stab, or quick clutch release.

There's not a bad suggestion for street tires in this thread, but honestly, you're not going to realistically see a great deal of difference in any of them.

If you go to a 17" wheel, which you state you don't like, you're going to be in the neighborhood of 2-3K after purchase of wheel and tires, assuming you go with something off the shelf like a 17" rally II, or Torq Thrust. For similar money, I'd possibly look at upgrading the suspension.

You've got some choices here. You can do a 4 link kit, torque arm, or do some upgrades to your factor leaf setup.

Going to a mono leaf with a cal-track or similar device and slider links over shackles is known to produce good traction. You also gain adjustability to set your anti-squat where you want it, instead of just where it is (often way too low with many leaf springs). The trade off here is that this type of setup can be noisy, especially with the sliders in the back.

With a 4 link like the Ridetech setup, you get the ability to again set your anti-squat where you need it. This setup is going to be more costly, but it's ride quality and NVH will be less. In either case, you're going to see enhanced traction with any type of tire. You may find that the 15" tires cope much better here. You keep the look you want, gain traction and aren't on a tire that you have to replace every 6-8 months.

Food for thought.