The car in Hardcastle & Mccormick was a modified Manta Montage. They had a couple of them, VW powered. Later in the series they replaced the car with another based on the DeLorean.
I've got a Manta Montage that I'm restoring. Tube frame, Mustang II front suspension, double-a arm rear with coil overs. Standard motor was a 2.8 liter V6 (130 hp). I'll be putting in a 3.8 liter supercharged motor (300 hp).
Pictures, history and such at my M6GT website:
http://www.birch.net/~petek/m6gt