I run the 45 side draft webber, It's the POO I tell ya.
Also depends on HOW Original you want to keep the car too. You should just be able to buy a webber from any auto shop, it's not a specific webber to suit a mini, they are all the same basically, you jet to suit the car.
BUT, Just find some one good to set up, balance and jet the thing specifically for your car. A good dyno shop should be able to do it, but, as with everything to do with mini's its best to find someone that works on em alll the time. they'll know all the tricks. I only run the short inlet manifold, but, the longer the inlet manifold the better the airflow, the more hp's you get. I once borrowed a 7 inch inlet, had to move the speedo out of the way, the webber basically sat inside the cabin with me, approx where the speedo used to be. Was running open ram tubes,(hadn't put the socks back on as well, it looked cool) sounded amazing under acceration, BUT, then I backed off for a corner, the thing backfired , and flames shot past my shoulder into the back seat.
I took long manifold straight back out and put back my shorty. It was never going to actually be run in the car, as.... well, I LIKE the speedo there, But it was fun trying it out. 
