The reason for the higher PV is to supply fuel once the load is high enough to warrant it. Of course you have to change it to see where it needs to be for your engine to get the best performance.
As far as timing, you need to look at the plugs to see what it wants for timing, being too conservative will lower vacuum and create more exhaust heat do to fuel burning outside the engine. With the timing correctly set, it will burn the air/fuel more completely and at the time needed to have the greatest cylinder pressure when the piston can do the work most efficiently, allowing for smaller throttle openings for the same power creating more vacuum. Without O2's reading the plugs are even more important as they are your biggest tuning tool. Sneak up on it a little at a time, but I would expect 36-38 to be closer to optimum for you. It's usually high efficiency heads like GM's Vortec or like my SB2's that live well at lower timing, my SB2 runs best at 29˚.