I believe you are looking for manifolds from 1957-1960 only. Check ebay, Wallace Racing, Butler Performance, Pontiac Safari.
There are both factory and after market manifolds available to work. Is your 2bbl a rochester or a stromberg ww? The GMC 347 had a large 2bbl manifold with an adapter for a small stromberg ww carb. A larger Rochester might due what you want.
My 59 GMC has a 59 Pontiac 389 that has been stroked to 447 with a 57 Pontiac tripower manifold. The trips are a great combination.
The easiest and cheapest thing to do would be to replace the 2bbl with the another 2bbl.
