Build-to-Fit tack:

All welded up:

1/2" piece was clearanced, this took a while going back and forth trying to remove minimal material. I needed it to be normal to the boss on the block with the 12mm bolt to ensure the pulley would run true:

I used a 10mm thread for the smooth idler that attaches to the bracket, I also had 10mm helicoil inserts so I used one of those. Here is the whole pulley shootin match:

I need a 5-rib 565 or 570 to try out, looks like this belt is hard to come by. Caveat here, I would strongly prefer to use parts off the shelf locally, so I am avoiding ordering online especially during development where return shipping costs 2-3 gallons of gas. After quite a bit of research, I found NAPA is stocking 6-rib 563, 569, 575 belts so I am fixing to head out there now. I also ordered a powermaster 6-rib 2" diameter pulley (ours is 2.2") and looks like it is a standard bolt on pulley I removed the pulley last night.

*Worst case I need to cut the 6th rib off one of these belts if my 6-rib conversion fails.