I would be more concerned about that 38 total degrees of timing than the vacuum. Most small blocks require around 35 degrees. Unless you have had the engine on a dyno and this combo wants 38 I would stay on the safe side at 35.
could these 38 degrees be fuelrelated? me and chrippa is from the same country and my 340 also seem to like 38 degrees, just asking because the thought crossed my mind..