You could use one of these types of fill tubes that has a push-in (with spring tabs) breather. chrome oil fill tube and breather cap,SBC,283 327 | eBay
The PCV would now be a problem though, as this kind doesn't have provisions for one. Maybe you can search and find one that has both a push-in breather and pcv.

 

Yeah, I thought about that, but I'm trying to keep the vehicle pretty-much entirely stock, so I'm trying to avoid that for now.

I checked for that opening in the back casting, and what I found was merely a circle in the casting but the opening was not cast, meaning there never was a PCV Valve in the rear of the block. Also, a test with the engine running revealed an excessive amount of negative pressure that could conceivably collapse a valve cover or an oil pan if it got any higher. So, I've ordered a vented breather cap and grommet, and will remove the L/H valve cover and drill a hole for the grommet towards the rear, inserting the vent cap into the grommet. Assuming I position it properly (so that the tube from the cap is situated between rocker arms and doesn't hit anything), this should provide a suitable resolution to the problem by giving the engine a point from which to draw fresh air into the crankcase (via the openings in the head) and relieve the negative pressure buildup at the PCV valve (which will still remain up on the filler tube). Not only will the negative pressure issue be resolved, but it should also actually be functional, allowing fresh air into the engine and ultimately sucked into the PCV Valve... taking the crankcase fumes with it.

 

How long has the engine been running like this?
You can't just drill hole in valve cover and put grommet and breather in, you need a baffle around hole or oil will just blow out into breather. Look at back side of stock covers with hole and you will see. I don't remember ever seeing engine with both valve covers not having holes in at least 1 of them,course I mayhave just not been paying attention, but seems weird cause engine has to breathe somehow.

 

I understand where you would need the baffle underneath the point where the PCV Valve is located, as it is prime to simply suck-up oil along with its air intake at that point. However, I don't see that you would need a baffle at the point where fresh air is being sucked-in above (or between) the rocker arms, since all air coming in at that point would then be immediately coursing down into the crankcase, as it follows a path ultimately towards the PCV Valve (the source of the negative crankcase pressure)... It is negative pressure at that location, not positive pressure, so the oil flying around at the rockers shouldn't be following an air-path outbound, but inbound instead. I say this because I was surprised at how much suction (aka: negative pressure) was actually present in the crankcase, at the time I observed all this earlier today.

And, I agree that the engine needs to "breathe", which would be the whole purpose of venting inward-bound air at the top rear of the engine. Theoretically, I would think this should work safely... unless there's something that I'm not quite understanding.

 

Did you buy car like this??

 

here is a pic and where the caption says (what is this) that arrow is pointing at the down draft system.
I need a bit of help... (Vacuum and PCV)

 

i put a freeze plug in mine and ran the new pvc,and breather.
the hose from this should just run down the rear of the engine and dump on the road.

 

Bob, the way you plan to do it will work fine, but you will want to baffle under the breather or find a breather that has it's own baffle. The crankcase pressure direction fluctuates as the pistons go up and down, and the breather on the valve cover will breathe in and out. The breather won't be drawing in like it was a vacuum cleaner. At a swap meet, I picked up a couple push-in breather grommets that are baffled and have a foam insert. Pretty slick and a simple way to prevent oil splooging out. You're lucky you didn't suck a gasket in, or had any other oil leaks.

 

I think the BLOCK was changed to newer one without down draft hole and nobody changed 1 valve cover to acomodate for breather.

BOB, since you don't have hole in block, just buy one new valve cover with pcv hole, and not oil fill hole, and just put 90 degree elbow and breather hose on and install fitting for hose on bottom side of air cleaner so that it can draw air from filter since that it were original down draft got air

 

I would tend to agree with you, in that the block was simply exchanged for an already-rebuilt engine, rather than the original engine being rebuilt and reinstalled, since the rear down-draft hole is now non-existent.

Do you happen to have a picture of the breather cap that includes its own baffle? I've never seen that before.

Otherwise, maybe I should just do what carguy68ssrs suggests, and instead of installing a breather cap into the valve cover, I'll simply run a hose from the clean side of the air cleaner to the valve cover (via an installed grommet). FYI, what I'm trying to do here is to avoid buying aftermarket chrome valve covers with factory-made grommet holes... as I want to keep my original Chevrolet-script orange-painted valve covers.

Oh, and yes, the car came to me that way when I bought it at a collector car dealers... believe me, I would never have screwed it up this way myself!

Thanks for the suggestions, everybody!