The nice thing about the MP distributor is the ability to adjust the amount of mechanical advance, the rate at which it comes in and the ability to adjust when the vacuum advance starts to come in. Mallory sells a nice tuning kit (#29014) for this distributor.
As it is delivered it has a fast advance rate but otherwise is set up for a stock engine. If you have a bigger cam, more compression or aftermarket heads it is not optimal and your engine won't provide max power if you just install it.
FWIW, 38 total is a good ball park number for a BB but small blocks with open chamber heads typically make max power in the 34-36 range and closed chamber heads like the magnum in the 32-34 range. And as others have stated the vacuum advance will improve economy and minimize plug fowling at cruising speeds.
If your car is driven on the street it will be spending 80-90% of it's operational time at light throttle cruising so it only makes sense to dial the vacuum advance in too.