There is no "alternative" 4 bbl. carb for a 56 TBird. All 56's used a teapot. If you are using a spacer than that could be your trouble.. 56 Tbird used a very thick gasket between the carb and manifold and that's all. Gasket is available from TBird parts suppliers.
The wrong air cleaner could also be the problem if you have a non-stock one. 55 and 56 TBird air cleaners are different (bottom part), mostly because of the automatic choke location.