[JLMounce]

The Global West pieces use a mix of spherical joints and del-a-lum or rubber bushings.

In a converging four link like that of the OP's 1986 GTO, the rear end must be able to articulate. Poly should never be used in the upper mounts because it can start to induce bind. Rubber is soft enough that it allows the axle to move in the directions that it needs to. For street applications where NVH is a concern, you always use Rubber bushings in the upper mounts (frame side and differential side) and at minimum on the frame side of the lower mounts. Poly can go in the differential side of the lower control arm.

Since this is a street car, with the Global West pieces I would run the following:
Upper Control Arms - TBC-82. Spherical frame side mount with factory Rubber bushing in the differential mount
Lower Control Arms - TBC-4. Spherical bearings on the frame side with del-a-lum bushings on the differential side

If you're not concerned about a little extra noise in the back, upgrading to the rear-end bearing kit SP-47M (for Moser) which changes the differential bushings to a spherical bearing, that will give you the ultimate in centering and articulation for the factory rear end.

It's important to note, these changes don't inherently give you better handling than what the car is capable with the factory setup and rubber bushings. What it does is allows the rear end to articulate properly and thereby gives you more tractability and linear progression towards the limits of adhesion. With rubber bushings and especially with Poly bushings, wheel rate is being added as the bushings take a crush and begin to bind. You're adding traction here until you reach critical bind, then the rear-end unloads and induces the snap oversteer that is characteristic of the converging four link design.

If you're doing the rear end, you may also consider going to coil-over shock, a sway bar and additional frame supports. I'd give Global West a call and talk to them about your needs based on how you're actually going to be driving the vehicle.