These belt guards are punched out with the original Whizzer dies from the 40's
Longer screws are use to fit the guard between side cover and petals.
I know how much you like to make your modifications the best you can and turn out looking as clean as possible.
I know how to mount that vintage belt guard with pan head screws so it looks OEM
Using what you have already made for mock up
1) Mount the adapter plate and belt guard on the engine and adjust for clearance, exactly where you want it.
2) Remove belt guard
3) Measure how much each corner was adjusted / Measure the distance needed from inside belt guard to mounting plate
4) Using coupling nuts as threaded spacers, cut/trim them to the length you measured in #3
5) Remove mounting plate from engine
6) Remove all nuts from the 4 screws
7) Screw the 4 coupling nuts to the mounting plate
8) Weld each coupling nut to the mounting plate
9) Remove the 4 screws from the 4 coupling nuts
10) Mount adapter plate to engine
10) Use pan head screws to mount the belt guard to the adapter plate
11) Use pan head screws the correct length so they won't go past the mounting plate and interfere with the belt
Pan head screws will be much cleaner and look OEM original
To have an acorn nut on top of a flange nut just sticks out way to far on the face of the belt guard.
Even using just an acorn nut, that still sticks out too far and your foot can hit it while peddling, besides it don't look OEM
There's more than one way to skin a Kat
Just some friendly fabrication info not to bash your original Idea.