Every so often I'll read in a thread that Without a PCV hooked up that a 351m\400 (or any engine for that matter) will buildup internal pressure within the block and result in oil leaks around the Pan gasket etc..............Question?? I've got a PCV currently hooked into my the base of my carb on my 400....and i also have a 5/8 inch hose routed from the base of the aircleaner over to the right side valve cover. At idle there isn't very much (if any suction) so I'm hoping that at highr RPM it will actually "Remove" some of the internal block pressure as it is Supposed too. Are these two sources of vacuum sufficient to keep the Block "Vented" and prevent oil leaks from starting?????fd

 

I don't think that the engine builds up too much pressure in normal use, and those two hoses work well. The old engines were just naturally vented ("road draft tubes"), not even using suction. I think blow-by will make some extra pressure, though.
My 5.0 Mustang had a mesh filter hidden below the PCV that got plugged up, and the engine started leaking everywhere (even around the distributor). So it does do something useful (I think that it also sucks off water vapor and evil fumes that will contaminate oil).
Not any facts there, just rambling a bit.