Joerg - That depends on a few things. Basically it would have been a single barrel Rochester Model B (if manual choke) or BC (if automatic choke - more used on car engines than trucks).
The Chevy truck 261's used a larger version of the Rochester B than the 235 and even smaller 216 versions.
You can check out Rochester carb identification info in several "Tech Tips" found
HERE.
There are alternative carbs from the Rochester, like the Carter YF series, which many folks prefer, but again you'll need to be sure you get one that at a minimum is for a 235 or 261 engine.
If you want simplified use/maintenance and tuning, stick with one single barrel carb. You can certainly go dual carbs, but that can become a real pain in the you know what to setup and balance.
On the used or new question, you will not likely find any "real" Rochester or Carter carbs, only used/rebuilt ones. The "new' stuff being offered looks pretty and close to accurate, but is all made overseas and more often than not is barely usable out of the box. They are known for very poor quality unfortunately.
