When Porsche developed the competition version of the 914-6 they switched from solid to vented rear brake rotors. These vented rotors, 901.352.041.12, are no longer available.
The most common solution is to replace the 914 rear rotor with the contemporaneous 911 rear rotor and then reduce the OD from 290mm to 286mm.
PET '65-'69 911, Fig. 6-03-05, #(1) 901.352.041.14 Brake disc, Rear, Vented, 911E,S (-'83) Qty: 2

Since the 911 rear vented rotor is thicker (20.0mm) than the stock, solid 914-6 rotor (10.5mm), allowances must be made to re-center the rotor in the wider, GT calipers.
The common approaches are one of the following:
* Move brake caliper inward by spacing it 5mm further away from the trailing arm mount
* Move brake rotor outward by spacing it 5mm further away from the hub

Both of these options are available:

Caliper spacer
Pros: Modeled after '68 911S rear caliper spacer, Lighter & cheaper than rotor spacer
Cons: Often requires notching the rear trailing arm for brake clearance

Material: Stainless steel, 304, 5mm
Finish: Mill finish
Coating: Zinc, Type II (Gold)
Weight: 61.12 grams
Includes: DIN933, Hex Bolt, Full Thread, M12x35x1.5, St, 10.9 (Caliper To Arm) Qty: 4

Cost: $52.24 pair

914-6GT_RearBrakeCaliper_Spacer_06_800x600.jpg 914-6GT_RearBrakeCaliper_Spacer_12_800x600.jpg

Rotor spacer
Pros: Increases rear track
Cons: Moves tire closer to fender, may exceed unused stud length

Material: Aluminum, 6082, 5mm
Finish: Ra 12µm (As machined)
Coating: Anodized, Type III (Hard, Grey-bronze)
Weight: 157.18 grams
Includes: DIN63, Countersunk screw, AM 6x35, SS (Rotor To Hub) Qty: 4

Cost: $42.56 pair

914-6GT_Brake_ROTOR_Spacer_04_800x600.jpg 914-6GT_Brake_ROTOR_Spacer_09_800x600.jpg

Shipping within the US is by the ever more costly USPS Priority Mail ($11.40). International shipping is available.

Other available parts:

911S_RrBrakeCaliperDogbones_04_800x600.jpg Tch,DN20_HoseFitting,Male_03_800x600.jpg 914-6IgnSwitchBezel_03_800x600.jpg