The Chevy 283 has a "draft tube" which vents the motor. It just runs from the back of the block, to the outside atmosphere (to the ground). No valving. I took a standard freeze plug and modified the inside diameter to accept a valve cover grommet. Press in the freeze plug, install grommet . . . and you can use a PCV valve (one way) to run a hose under your carb and keep that negative block pressure we all love so dear . . .