I think Lunatic has it opposite. He's reliving tension on a stock set up for some reason, so the tensioner will just rock back to the loosest position of the holes he hogged out and stay there. Would work fine for that purpose.

I think with the sprockets you want to put on, the belt will be too loose and you will need to take up slack in the belt and fight to keep enough tension on it.
I've done that both with belts and chain conversions on Buells and haven't found the clamping force of the nut on the stud to be enough to hold proper tension on the belt or chain, even when over-torqued. That's why I recommended a mechanical, adjustable stop like a big set screw.
In this configuration it would be pointless to slot both holes and make the whole bracket wobbly. Just slot one and pivot on the other one.

YMMV, IMO, TL;DR, LOL