I had the same problem on my 62-235 c.i. engine. Took the manifold to a machine shop and found that the exhaust manifold had to be replaced as it had been milled too much. Found a used manifold and took it apart for milling. Put the intake and exhaust back together but left the two parts slightly loose until they were installed on the truck. Then tightened everything down...no more leaks!! Hope this helps.

Green Machine