What is the best way to determine gear ratio?
Of what? the rear axle? If so, jack up one rear wheel on a non-posi car, put a mark on the tire and driveshaft. Then rotate the tire 2 exact turns whil counting how many turns on the driveshaft. If the shaft turns 3 & 3/4 turns, it's a 3.70 ish; if 2 & 3/4 it's around a 2.70 ish.
If it's a posi, jack up both wheels but only turn the tire one turn.
The old speedo should work. I ran a 30 DeSoto with a GM 4 speed trans. The fitting on the old speedo was the same as a more modern car. My old speedo was not a needle, rather a rotating dial.