That's not really a cruise control unit, just holds the throttle in one place, which might be fine if everywhere around you is flat. Have a look for motorbike cruise control units, or see if you can find an Audiovox CCS-100 (or clones) for sale locally, I know someone who fitted that to an old Volvo 240 and spoke highly of it.
Current fleet: Triumph Dolomite Sprint '75, Daihatsu Fourtrak, Honda CG125, Yamaha Fazer 600, Shetland 570
Disposal fleet: Golf GTi 16v MK3 Anniversary
Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated it on pickup from dealer, was made out of magnetic metal as only car I've ever been crashed into, 4 times), Golf GTi MK3 16v x 3