A few questions and a few answers.
The first thing was what year car he has, but he answered that with this photo:
There were only three carburetors used on 318 2-bbl FMJ's: The Carter BBD, Holley 2280 and 6280. The 6280 is computer controlled along with some BBD's.
He has the Holley 6280, based off of his pictures.
The Holley 1945 is a single barrel used on /6, so not applicable in this case.
This is a picture taken from my '86 Fifth Ave (also showing a Holley 6280). It looks to me that someone changed carburetors at some time and/or changed the intake vacuum tree for the brake booster. I think the former is more correct than the later.
The brake booster, according to my vacuum hose diagram shows to hook up to rear of carburetor in addition to the PCV hose:
if this isn't the original carb, that port is unused and needs to be plugged. I'd get an actual vacuum plug instead of a bolt shoved in a hose though.
I do agree with this statement, Go to parts store and get a rubber (or soft plastic) plug and stop this port the correct way. That hose will split and will cause a large leak sometime in the future (sooner than later).
The port is 11/32” (0.3438”) which is not a common size. 5/16” = 0.3125” and 3/8” = 0.375” (which is a tad too big) are common sizes.
Actually. I would move the PCV hose back to the correct port – but that might not matter.
This picture (green circle) shows the disconnected “vacuum T” that might be the source of running rough. There is a hose going to air cleaner (heated trap door, over drivers side exhaust manifold, that connects to that “T”). When removing the air cleaner, I prefer to disconnect both hoses at the air cleaner. The one mentioned in sentence above, at the air cleaner temp sensor and at the computer. I then plug both with a Phillips or Torx screwdrivers – so engine "could" be ran with air cleaner off (which is hard to do if engine computer was disconnected, as well). That way, I recover my screwdrivers and pretty certain everything is re-hooked up before closing the hood (or what method works for me, anyway).
Another reason I disconnect the hoses at air cleaner/computer, is because heat causes the hoses at carburetor to get hard faster. the hoses at other end stay soft a lot longer (from lack of heat?) - so fewer chances of hose cracking/breaking. It is also faster for me to disconnect/reconnect at both ends.
The yellow arrow is your EGR valve. I would bet (a dollar) your passageways inside of your intake are 100% stopped up from carbon deposits – which would render the EGR valve inoperative,
IF the EGR valve has been disconnected for a couple of years or longer.
BudW