Well first you'll have to buy some twin carbs, which don't usually come cheap. But after that, as long as you have the manifold and the linkages, you should be away (after a good tune).
If you want to try another option, you could grab a weber from another car like a cortina or something. I know the XF falcons came with 34ADM Webers which seem to make any engine up to about 1600cc run pretty sweet. A shop like Weberspecs should be able to sell you the adapter plate / manifold, which ever you need.