Dave's page lists the stock diff ratios:
http://geocities.com/MotorCity/Downs/5134/
The ratio may be different but probably not off by that much. You didn't mention what year your Z is or what you're getting the manual trans out of so I can't say exactly.
I ran my car with probably the most extreme mismatch you can have - an '82 5-speed with a 3.36 diff, and it was very driveable. A little slow off the line, but not horrible. So, it should "work" no matter what diff you currently have. If you don't mind going underneath there twice, you might just change the tranny and feel how the thing drives. If it's too sluggish off the line, get a higher ratio. If it's too quick off the line and revs are too high on the highway, get a lower ratio.
The only way to tell what ratio you actually have is to pull the back cover and look for the numbers stamped on the ring gear (such as 39:11 for a 3.54 diff) or to put it in neutral, lock one wheel, and spin the other wheel while counting driveshaft revolutions.
