Whether you fabricate, or find something close that will work you should be fine. I would try to keep the i.d. close to original, and the added length down as much as possible, but it should all work with no problems. Don't forget to isolate the carb in some manner frorm heat however. Even if you just use a plastic, nylon, delrin, or a resin/phenolic spacer. I once made an isolator out of 3/8" plywood coated with epoxy. If you don't, the heat soaked carb will cause the engine to be difficult to start once it is warm.

AV8TOR