323 355 or 373 gears??

The Gear vendor unit allows more ratios without having to make multiple transmission modifications. Build the trans strong with lower ratios and use the Overdrive to get the final ratios that you want. The trans would fail well before the Dana or the Gear vendor units. They are designed for hard core torque the 4L80- is not.
I included trans, tire and mph calculators and a differential width chart.