I would think that's a standard EGR port, but could be wrong. If you find one listed for a different application, you could check on Rock Auto and see if the use the same EGR gasket. If the gaskets are the same, the block off plates should be the same. No one is going to market a block off plate for the few carb/turbo V6's we have.
On the '83 plenum, the EGR passage provides heats (which is why it goes front to back). Without it, you could have fuel puddleing issues under the carb, which will lead to a bog when the ambient air is cooler. The EGR, EFE grid and THERMAC air cleaner all kept the things warm for better atomization of the fuel. I'm not saying to keep them, but realize the trade off in cold start drivability when getting rid of them.
On my '83, when the EGR valve went bad, I found that the knocked a lot when cruising down the highway. I know it sounds counter-intuitive, but EGR helps suppress knock at part throttle. At wide open throttle is shuts off, so not much is gained by removing it.
I am getting rid of the EGR on my '82 GN, but that will be a track only car. I'm using a 1980 plenum that uses coolant to keep it warm. I think this will control heat better (not so hot and more consistent) than the EGR set up. The EGR setup in 1982/83 heated quicker for better cold start emissions.
What are you plans for this engine?