Take a look at what the Mini spares suppliers offer-
Try Somerford Mini, MiniMachine, MiniSport, Mini Spares, Mini City, Play Mini etc. Or try Burlen Fuel Services, see what they have to offer.
There was a thing called a 'Torque Master Manifold' that was for single SU's it was claimed to be the business over the twin carb set ups.
Cheers
Peter W