At the risk of ruffling some feathers, I'd have to say that a bolt-in pre-fab cage is not a good idea. That's especially true for Z's since NO MAINSTREAM HOT-ROD MANUFACTURER makes an NHRA-approved "kit" for the Z. If you call Morrison, S&W, Magnum Force, Steve Alston, whatever, they will sell you a generic package of tubes designed for a Vega or Pinto. They don't fit well, and they are plagued with unbraced bends in the tubes (prone to buckling).
A real cage is A LOT of work. You end up stripping out the entire interior, the carpets, plastic body panels, everything. You clamp the unibody to a rigid chassis jig, so that the thermal stresses from welding don't cause displacements in the metal. Then you have to worry about where to anchor the tube ends. Sheetmetal? Forget it. You need reinforcing sub-structure underneath the cage. For maximum benefit, you need to weld a network of gussets to secure the cage to the room, B-pillars, etc. And you will end up with diagonal members barring the door openings and criss-crossing the car.
IMHO, if this is not what you want, don't build a cage. (My appologies if I'm starting to sound like Mark Sayer )A hoop behind the driver's head might save your life if a tractor trailer rolls over the rear hatch, but it won't do much for the structural integrity of the car - especially for torsional stiffness.
BUT - I've seen 11-second V8 Z's run at national drag racing events with simple roll bars and subframe connectors. We're not building 1200 hp pro mod dragsters here (and frankly, I think that these pro-mod dragsters are not very well braced either). If this is a budget V8 conversion, if you spent $500 of a Z shell and most of your money is going into the engine and drivetrain anyway, it makes more sense in the long term to just let that unibody twist and then discard it and to buy a new one. Building a cage becomes like paying more for insurance premiums than the cost of what you're trying to insure.