OK. Looks like I need to try the "counting revolutions" method of the tire and driveshaft. I think I understand how I'm supposed to do it. I determined that I have an open rear end because as I turn one tire, the other turns in the opposite direction. So I need to block one tire, mark the other relative to a stationary reference point on the car, and also mark the driveshaft relative to the diff housing. Then count the number of driveshaft and tire turns it takes for the marks to line up again. Then (driveshaft/Tire)x2 is the ratio:1. Right?

I have a list of the common Pontiac ratios. Does anyone know of if the chevy ratios are the same?

From there, I think there's more math to figure out the speedo gear I need.