Phil,
No argument from me with your statement that you can switch the B18s and B20s crankcase ventilation routing. I'm concerned, however, because I've been telling people something different (based on the manuals I have) from what I understand you to be saying. As far as I can tell, the original B18 set-up, with the PCV valve atop the little pipe coming out of the teardrop, has no flame trap there, although it has one in the oil filler cap. The original B20 rig (teardrop connected to air filter) does not include a PCV valve and has the flame trap (rolled up brass foil) inside the pipe coming out of the teardrop. The pipe coming out of the teardrop on the B20 is fatter, to accomodate the flame trap. I rigged the B20 in my PV as a B18 because I used the B18 carbs and manifold, and to be able to use a PCV valve I had to fit in a transitional piece of hose so I could get that PCV valve to match up with the fatter teardrop pipe -- the B18 teardrop doesn't fit into the B20 block, at least, it didn't on mine which used to be a B20B. Of course, all of this may have been different at different times, as is not unheard of with Volvos ;-).
Bob