I haven't read all of the responses above, but my first thought on this is the original engine in any cab over is going to be a low oil pressure primitive 235, not a 216. Any two ton truck was going to have a 235 until they came out with the 261 later on. I know the 216 intake manifolds had smaller intake ports than the 235's. I wonder if you have a 261 and the intake manifold is just too small and not able to move enough CFMs. You are working with the original UPDRAFT caburetor and manifold system correct?
