Rod bearing sounds like a plausable diagnosis. If the crankshaft is not damaged, the fix is easy. Drop the crankcase pan, locate the bearing and replace it. Takes longer to drop the pan than to replace the bearing. If the crank is scored, it will need to be removed to be properly repaired. Radiator comes off, clutch comes off, then crank is removed. That's a lot more work, and I doubt if the mechanic will do it anyhow.
As far as the oil pressure, 30 is darn good in a 235 at idle. The lack of pressure at times is a major concern. The problem is usually worn bearings, allowing too much oil through. Be sure the mechanic hears about your oil pressure problem, too. It doesn't sound like he did much of a rebuild on the engine, in all honesty. That's why we encourage people to do the work themselves on these old sixes, and many of us will even provide on-site instruction when needed.
