[clay68c10]

Your gearing should be fine with the 4L60e. 3.07 would be a downgrade. Look at some online gear/rpm calculators and compare what the stock LS equipped trucks run for rpms at different speeds. I bet you're close to that now.
Get the right 195 thermostat in there, it's probably not helping you mileage.
Those low temp thermostats don't fix anything, these engines are made to run around 200-210. The stock fan settings are higher than you would think, like low at 210, high at 220.
Your system voltage is low, 13-14v at idle is normal. I'd say your guy is right, it's new alt time.