Another thing is that the 216 and 235 manifolds have a different bolt pattern when bolting to each other. I have not heard of this causing fitting problems at the flanges though, just the need to slot the inner 2 holes on the intake. Not sure if the angle of the mating surfaces is the same between the 2 types or not. Try what J.L. is suggesting first...
Did you try a 1/8" npt fitting? the two are very close to each other in size...
