That application would be better with a built TH400. Any gearing more than that 3:70 will be revving too much on the street and you'll hardly get it out of 1st gear.
My 69 Camaro was originally running 28 tires with 4:11 gears and TH400. I changed to 3:70 gears to get the finishing revs down. It still had plenty of torque off the start line with 200hp nitrous and the finishing revs dropped 500rpm. It was a hellova lot more street friendly too.
I think you have it pretty right. Just chuck in the TH400. Much better on the street.