1998 Chevy Cheyenne Timing after intake gasket replacment

I replaced lower intake gasket on 4.3, and set the distributor back to what I thought was 0. It ran rough and had a knock, I reset the distributor again and it ran better but still had a miss but no knock. I took it to a mechanic and he said I drop something in the intake and it fell into the # 6 cylinder causing it to knock. He says #4 and #6 has misfire and need to remove head or get a new engine. I forgot to metion that I also replace distributor. Am I wrong to think that the mechanic is wrong and that he does not know how to do a crank relearn. Or the knock and miss really is caused by some misterious object in the cylinder?