I found some pics of SUs on the web and stuff, and i think that they are the 38mm units. They are the standard ones off a 1600, thats what the bloke said when i boughht them. Ita also helps that they came bolted to a 1600 manifold.
I also have a smaller type Su with the flat top domes. They would have to be about 20mm i guess. They came of a 1000 from my friends grandfather!
Another question would twin 38s be too much for a slightly more than street 1200.
( ported/polished head, 2mm shaved block, really lumpy cam, headers, 2"straight thru exhaust)