Need advice for slowing my car down consistently.

It would only help a little bit, but there's always the option of adding weight - approx. 100lbs per .010. Of course you can only do so much and it needs to be done safely and legally. Best thing about it is that it would help quite a bit against spinning!

Less gear and/or taller tires? But that's getting expensive.

I was thinking weight. Lots of weight...lol. Easier to be consistent with the package. Taking out timing or partially shutting the carb will change a lot and like Loco said consistency will suffer.