I notice you said that you cruise at 45-55 at 2-3k rpms. can you change out your rear gears? it was discussed once before on here how to modify a miata. (actually they guy wanted to put a VX motor in one) but I suggested changing out the rear. I did some looking and found out that there aren't that many choices but if you previously used this car for racing, I figure you have some prett deep gears in it. also if you used this car for racing, you may have some contacts that we don't.
I would check the rear gears to see what gearing you had and maybe go down a notch or two.
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