I raced a 75 280Z in ITS for 5 years. I never encountered any issues with the halfshafts running an R200 running the car with the GC coil overs and at minimum ITS height.
The R180 would fall into a "grey" area of the update/backdate rule. It doesn't specify that any of the 280's were ever equipped with the 180 from the factory, so therefore on the spec line for the 280 only the R200 is listed, since automatic cars are not allowed in IT, they wouldn't have included the later 280's that came with the R180 diff.
In my mind, you will be better off running the R200, due to the extra weight the car has to carry anyways.
I ran mine with a 4:11 that was welded, and never had any trouble.
The Z/ZX 5 speed question is probably one that no one will ever protest.
My car weighed right around 2700 with a 2 inch Autopower weld in cage if I recall correctly. This was also long before full gutting of the interior was allowed in IT, so making minimum weight now should be fairly easy if you build a well designed cage of the minumum diameter tubing.
As far as protests are concerned anymore, if you race in a region where there are some dominant E36's such as the DC region has, you should have to worry about anyone protesting you on the diff or tranny as you'll have a tough enough time trying to run in the top half of the pack today. When I raced the 280, I could stay with the top running 240's and RX7's in the turns, but couldn't keep up on the straights due to the extra weight and the handicap of the stock FI.
Of course, if your region has an SPO/SPU class, you could always build the car the way you want with larger brakes and yank the FI in favor of the stock SU's, cut a lot of weight off the car (those bumpers are anchors) and just go have fun.
