Exhaust Vacume question

You ought to have at least some air entering the crankcase to sweep it clean and prevent dirt ingress past shaft seals. How to set it up depends on how tight a ring seal you've got (i.e., how much pressure there is in the crankcase). Be careful how much vacuum you create in the crankcase. Too much and you'll suck in the longer gaskets (oil pan, valve cover) and create oil leaks. You may want to put it together with the inlet breather(s) blocked off, rig up a vacuum/pressure gauge to look at the crankcase, make some test passes and see what the crank pressure is doing. If it's going way into vacuum, you can lessen the vacuum by means of a calibrated air inlet to the crank. Not too hard to rig this up if you think about it.