Hey,
My 76, 350, has a push in PCV vent on the left valve cover. Is it better to have it venting to the air cleaner? or is this pcv push in vent ( its the kind that looks like a mini aircleaner/ open element thing) okay?

WROR

When I built my motor I had two air cleaner style valve breathers and when I got it tuned up the mechanic said to put a pcv valve in it and run it to a vacuum source (or in your case the air cleaner if its stock) That way the crankcase is under vacuum and any gases inside will get sucked back into the combustion chamber and get burned. Also keeping the crankcase under vacuum will help the piston rings and gaskets seal better. So put the pcv valve on.

You should always use a pcv valve.

I have a pcv valve in the other side? Should i replace the push in thing anyways?

What you should have is a PCV valve in one valve cover and an air intake in the other. Factory config has the intake vent going up to a filter pad in the air cleaner. This is called a 'closed' PCV system and is used so that if you have a situation where blowby exceeds what the PCV system is taking out, the excess fumes (which end up going out the PCV intake) still have a shot at being burned because they end up inside the air cleaner. When you change to a push-in intake you become an 'open' system, where excess blowby (if any) gets released to the atmosphere. From an emissions standpoint not quite as good, but it doesn't really make a meaningful difference until you have signficant wear on the engine.

Of course, the valve should stay. But I think you are asking about using a push-in breather for the other side. In principle a push-in should be fine (I run one myself since I changed over to an open-face air cleaner). The thing you need to make sure of is that the filter media is okay. Some of the chromed push-ins have almost nothing inside them for filtering. Mind you, the OEM in-aircleaner PCV intake filter pad ain't the best one known to mankind, but this is air passing through your engine so you want to at least catch any inbound rocks.

If the PCV valve is working properly and you don't have excessive blowby, then the vent is intake into the crankcase, not exhaust from it. If you do have excessive blowby, you will want to address it before going to a push-in or you are liable to spit oil out onto the valve cover.

You don't want to just plug the hole because then you will pull crankcase pressure too low and will start introducing air from wherever any can leak in (around the oil filler cap, etc.). Better to have a filtered source.

Also, if you are in one of those areas that are rabid about emissions equipment for your annual inspection then (subject to the will of your inspector) you may have an issue, since the PCV system is considered part of the emissions controls in addition to it's other roles and an alert inspector will notice the conversion. Just for your awareness.

The system that Gozer describes will work fine. One of the reasons that you want a PCV valve is to keep the engine compartment clean. I'm sure you have seen engines with two intakes, one on either valve cover and no PCV valve. That system will vent all of the crankcase fumes into the engine compartment. The fumes are very oily and this oil forms a film on the engine and throughout the engine compartment which attracts dust, which turns then into gunk. On some cars the oil fumes from the engine compartment are drawn into the low pressure area at the base of the windshield. The windshield then gets deposited with the oil film and gets dirty and grimy very quickly. Use a PCV system, it won't cause any performance reduction and will keep your engine clean also.