So i bought a Z that needs engine rebuilt, which i am doing. I only drove it a little prior and couldn't tell any issues with transmission. However, I am considering rebuilding the transmission too.

SHould i go ahead and rebuild as i have most things already out or is it one of those don't fix it if you don't know if its broken things?

I assume it is a manual transmission.  If it shifted smoothly thru all the gears, and did not exhibit any excessive noise or whine, especially in 1st & 2nd gear, I would leave it alone.  Replace the seals, drain and refill with AC Delco Friction Modified fluid.   In addition, refresh the shifter bushings.