An easy check of the voltage regulator is to look at both the fuel gauge and temperature gauge -- they're both dependent on that voltage regulator, so if the fuel gauge is working properly, the voltage regulator is not the problem.

You said that you changed the thermostat, but perhaps you installed the wrong temperature 'stat -- and that presumes that it was of good quality. There are three temperature versions: 87, 89 and 92 deg C.

I'd suspect that you installed the 87, which would (and should) keep the temperature gauge's needle low, instead of near the middle. The 89 will keep it nearer the needle (now that you wisely removed the TCB), and the 92 will keep it slightly above.
BTW, the 87 is really too cool for efficient fuel economy -- the 92 is best for that, if you're not in a hot climate.