If the holes line up nice I personally would try to use the speed nut type like pictured even if you have to get bigger or even smaller ones.
Length of the screw is critical using those though, but I'm sure you can see what your limits are behind there where the screw would extend.
I remember doing one once with 10/32 machine screws with fender washers against the shroud and a nut in that pocket on the radiator frame.