Okay, go ahead and flame me but I know some of the members have experience with small block Chevy motors. And no, I am NOT going to join a Chevy forum to ask one question. With that said, the early SBC (early/mid 60's) PCV oil separators that went in the lifter galley, how effective are these at removing oil mist? Please don't ask why I want to know, I'm having one of those moments!

No reason to get one's panties bunched up over a Chevy question, IMO.
Physics applies, regardless.
Can't really be too informative on that lifter valley can, but the Gen1 SBCs were prone to be leakers, especially intake areas & valve covers.
Some of that seems to me would be crankcase pressure.
That lasted into the '70s - there's one from '79 (with a Targetmaster crate motor) in my K10 plow truck.
But decades ago on my '73 Z28, after going through it and getting serious about leaks, its motor was dry up top and didn't have that separator.
It would weep slightly at the rear main seal; the K10 weeps at every seam.
Stock PCV, no pan evac on either example.

Not very, some of the mist will get through and eventually coat the inside of the down draft tube leaving little drips here and there

Those cans located in the valley are there to keep the big chunks of oil from getting out through the draft tube, or the PCV valve when so equipped on the mid to late 60's engines that pulled air from the vent at the rear of the block next to the distributor. They worked adequately from my experience after seeing what happened if the under valley can was left out either by accident, or because of a person not knowing why the separator can needed to be there in the first place. When a PCV valve was hooked up to the rear vent without that can installed, an instant smoke screen occurred. Never saw what effect a can being left out, or if I did I didn't notice, any smoke or oil loss on a car equipped with a road draft tube.

No flaming you from me. I have been building a street /rat rod every year to sell for the past seven years. I decided to put a chevy 350 in the one I’m currently working on. The reason is they are easier to sell. All the potential buyers around here are chevy duds and they walk away if it doesn’t have a sbc or ls in it. Can’t change stupid so I decided to play with stupid. It’s all about selling them. My stuff is all MOPAR powered except one it has a 5.0 ford with a 5 speed manual in it. It’s for sale.

I'm guessing you're looking at an aftermarket intake manifold and discovered it hits the top of the separator. I'd run into that at least a couple times.

Otherwise you set it aside when you disassembled the engine and now can't find it. Done that too.

The down draft tubes don't pull the kind of vacuum a PCV valve will on a healthy engine. Early ones used to have a rubber grommet that fit where the downdraft tube hooked up, had a right angle fitting in it which would hook up to a PCV valve using rubber hose.

I was always concerned about pulling oil out of the valley, so would put a frost plug in the hole mentioned above. Remove the separator. Use aftermarket valve covers with the PCV hookup, but make sure there's a baffle on the inside of the valve cover. Some of the cheap ones don't, and I've had trouble with them allowing oil mist to be drawn out of the valve cover.

Hope this helps.

Thanks for all the good advice since this is my first sbc build I need all the tips I can get. I’m in luck because my buddy down the street is a diehard chevy racer is helping me. He use to own a race car fabrication shop in Florida and has built hundreds of sbc engines.