The TPI of the 5x0.8M screw is about 31.75, so for threadings less than 15-20 threads this
discrepancy is negligible. The OD of the 5mm is 0.196" and OD of an 8-32 screw is 0.163"
so it seems like it would be a bit rattly in the hole to use a 8-32 in a 5mm hole. Some
teflon tape around the threads might help. You can try it and see how you like it but since
the thread engagement is tip to tip and not tip in valley, you would find it easier to strip out.
Since only the tips are involved this would not do terrible damage to the female part if you
went back to 5mm later. There are versions of loktite that can fill gaps that large but they
are not going to add a whole lot to the strenght for short thread engagements like WB mount
holes. Fork mounts can be up to 6-7mm thick on some bikes which would be stronger.

Now a 10-32 screw is much better with a major diameter of 0.190 and should work very well.
Check Sheldon again to see if he meant 10-32.