Should shoot 168's fine.
This is why I would hesistate to change anything until AFTER I had shot the gun first. Need to get a baseline to figure out if the problem is the gun, or the way it was re-assembled.
Sometimes they will prefer one load far more than another, but 3.5" at 50 is beyond terrible. Have you looked down the bore, made sure there are no burrs, the crown is good, etc.?
You mention free floating the MI rail, isn't it free float as-is?