Best Way To Repair Butterflies

Solder!
Cuz IF, it falls out, it is more likely to pass thru the engine harmlessly.
But I've never gad solder fall out.
I chamfer both sides of the hole first, almost right down to the center of the material. I clean it, install a pellet,Crimp it in, and hit the plate with the heat, until the solder crawls in and makes a clean bond; one caveat, aluminum has a very low melting plate! When using a butane pencil-torch,keep it moving. I have used an electric gun as well, but it takes a long time to get the heat in there.

On another note,your sync looks pretty close, perhaps a tad small, but close. What rpm were you achieving at what idle-timing, and what are your cam specs and displacement.
But yeah, those holes are pretty humongous,lol. I had a 292/508 cam in my 11.3Scr- 367 for one summer, and it did not need that much bypass with just 14/16* of idle-timing.That was a 750DP. I'm still running that carb,now with a 276/110cam,10.9Scr, and the holes are soldered shut and filed nearly flat.