The 350z has low cruising rpm is because the 350z diff (3.54)is even taller than the TT /q45(3.69)diff. (Remember the NA 300ZX is 4.09 ratio.) Therefore even slower acceleration.
I’ve seen a bolt kit that allows you to bolt up the 350z diff (r200/v) to a 300ZX Na sub frame but people that do this , have TT swapped the engines with significant horse power increase. Here is the link to the kits.
https://us.gktech.com/s-r-chassis-to-35 ... n-bushings
Maybe swap both 350z trans and diff with a custom drive shaft???? The only other issue I see is speedometer offset if you change the final drive ratio of the car.
Hope that helps,
Peter