Firstly, you never lift a Z using the frame rails under the floors - even if they were brand new.
Secondly, Whenever possible use Granny's advice and lift the front using that big plate in the center of the front crossmember.
Thirdly, When using a floor jack (not a scissor jack, not a bottle jack), we lift the differential in the rear and then place jack stands. There are a few "hard points" in the rear to help.
Fourthly, The 280's added a lot more heft to the unibody and were strengthened in specific spots underneath (meaning: there are big beefy steel plates under there that the earlier Z's either didn't have - or were not as strong). You should learn those spots, they will help with lifting and supporting if you need to work underneath.
Fifthly, be absolutely certain of your jack stands if you're working to replace the transmission fluid. In most cases both the fill plug and drain plug have been over-tightened and many people have struggled to get them loose. That means you're putting a lot of torque into it, may having to use a "cheater" for leverage - you don't want the Z to come down on you while you're under there.
Sixthly, When you put the plugs back in, don't over-tighten them! There is no pressure on the fluid inside. Use Teflon tape (or anti-seize) on the threads and only tighten them to snug. You just want them just tight enough so that they don't leak - and done.