Spax makes a special rear unit for the long-wheelbase classic Mini models (the Van, Estate, and Pickup), which gives more length and travel. Gas-filled, with 14-point on-car adjustability. Should be fitted in pairs, but sold each. Ref: GM3, C-STR308.
Note from Seven: Unfortunately, at this time Spax is the only manufacturer offering shocks for the rear of the classic Mini Van/Estate/Pickup rear models, so they charge a lot for them and the retail is higher than the corresponding shock on a classic Mini Saloon (sedan).