All Plugs Fouling on Right Bank

Different application, but, I once saw a copper ball stuck in a hole and smashed sorta flat, so that the copper overlapped the edges of the hole.I can't imagine that ever falling out. It mightabin a BB.
I imagine you could employ the same idea with a bit of copper-wire;just be sure to chamfer the holes-both sides. It will be a bit tricky on your plates because of where exactly your holes are, but I see it as doable; just don't bend the plates!

You may be questioning the why of closing these holes.
Well, with air going through the plates instead of around them, the plates can be closed further to achieve the same idle-speed. But by closing the plates too far, the idle mixture being discharged from the transfer ports becomes insufficient for idle-speed operation. To compensate at idle, the mixture screws can often be tweaked enough to sorta make it idle. And if timing changes are employed, pretty soon it idles ok. But as soon as you just slightly open (tip-in),the throttle,everything takes a dump.First you may get a little stumble, and then it may go rich, on account of the mixture screws being open so far. Once the air is moving things settle down.That's why those little tiny(lol) holes gotta go!, to put the plates back in a proper working relationship to the transfer ports.
And as previously said, those holes are, for a 273, actually pretty big. It is surprising that the idle-speed isn't excessive. Are you running a PCV? If no then that would help to explain why someone might have put those holes in there in the first place; they are replacing the controlled air leak that the PCV system was providing. If you do close up the holes,and you do NOT currently have a PCV, then you will need to install one.But post #15 shows a hose going off the front of the carb,that seems to indicate a PCV off to the left.