"full 86 drivetrain swap (came like this)" fooled me at first... okay, so thankfully, no EFI, lol. I wish some folks would be more specific and add some more detail when having issues and asking for help over the internet. I hope you're talking about wide bands being nothing but an oxygen-sensing tool, because that is all they are, and can be fooled even with rich misfire and indicate a false lean condition... and they're as accurate as their makers, at idle or anywhere else, sensing the oxygen in the exhaust, and that's all they do... because you're talking to somebody who knows a carburetor can be dialed in to be deadly and super accurate... and 12.5:1 @ idle doesn't cut it in my book. 13+ at idle should be a breeze.
Besides the point for right now, but what's the carburetor? More on that later maybe...
The point: Reading the other comments so far... the previous owner "did" the rockers, did they?... ask the previous owner if any cylinder head work was done. You might not like hearing what I have to say/suggest next, that quite likely might be the problem, but don't kill the messenger that's just honestly trying to help you, lol... I've witnessed what you're describing once before, so it's pretty specific and is ringing bells here. The noise you're hearing sounds exactly like what valves, specifically and most likely exhaust valves, "hanging open" sounds like, maybe probably not until, or more so when the engine is warmed up (?), from valves (or new valves?) in (new?) guides that were not honed for proper valve stem clearance. You won't see exhaust valves hanging open on a vacuum gauge, but you'll hear them out the exhaust... a popping or huffing or puffing sound out the exhaust. Better pull the valve covers off again, and see if you can catch any of them with what seems excessive rocker arm to valve stem tip clearance, "hanging open"... and/or if this is the case, and it sounds like it, and not too much damage has happened already to valve heads etc, remove the cylinder heads and disassemble, keep accurate track of which valve goes back with which seat, like with a length of wood to stand against the wall/bench with numbered holes to stick the valves into while you continue to work, and then check and then hone or have the guides honed for correct valve stem clearances. Or have this done or parts repaired/replaced somewhere where they know what they're doing... and then everything might be much different and you'll be happily motoring down the road. Good luck with it.
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