I think it was fine threaded so 5\16 - 24 don t quote on that. You only need to adjust that table every blade change, so bi anually maybe. I would go with an allen head if you cant get square head.
I think the one foot square units are swamp cooler type, not good if you have any humidity. Likely they are as good as putting magnets on your fuel line and getting 50 mpg was advertised years back.
I would make the lowest shelf a bit higher than the unit in your picture. Bender over to get heavy boxes only gets harder each year. Maybe put open shelf for overstock items that need to be accessed only once.