This is NOT an easy mod to do! Specially when not having good reference photos of how the rally brakes were implemented by the Porsche factory. Starting at the front at the 'extended' hand brake-lever relocated between the driver and passenger seats, the 914-6 GT rally cars used a 911 M Caliper at the rear, a new set of custom-length brake cables and an adapted 911 drum-brake parking-brake mechanism to the rear 914 trailing arms, but using a pair of "911 M Calipers" instead. The 914-6 GT rear calipers are not used and will not work for the GT Rally rear brake conversion.

Also making it a bit more complicated is how the rally cars reversed the ratchet/pawl mechanism of the parking-brake lever. In this case, the pawl is never engaged to the ratchet on the rally cars. In order to activate the brakes, you would have to depress the brake lever button to have the pawl engaged the ratchet. We're still trying to figure how the brake cable was terminated in the factory GT Rally cars, particularly how the cable was terminated,attached to the brake-shoe spreaders https://pbase.com/image/167143249. A work in progress...