I've used adjustable diameter pulley, but to get the belt on where I use it I can make slack and then take it up again. That is the rear wheel in the drop outs, I move the rear wheel with pulley mounted to it forward and then back in place aft, after belt is on.
After a few tries making the adjustable diameter pulley larger to get the right amount of tension, then I have the rear wheel pulley in the drop outs and all is set to the right tension.
I think I see it is the one belt yet to have an idler for the engine to the jackshaft. That would mean that you have to make another idler in your construction?
Anyway just throwing that out there, don't know if it is of any use in the circumstances you found of current.
I know some mail order (but with minimum order like $75) that have 1/2 inch increments on A section v-belts. There is also Link type section belts that are A section. Those though have a limit to the amount of tension they can handle.
Popping the belt over the edge of a washing machine pulley attached to rear wheel spokes is what I have done in some cases, but it stress the belt and the pulley. The older steel large diameter pulley I had ages ago was good and would not be harmed, but cast white metal was all I could get for free. Even if I were to buy steel pulley if it still existed today it would cost more than the Whizzer clone sheave. The washing machine white metal cast spoked pulley mounted on my rear wheel cracked. I had an extra, but it eventually wore out and I got a Whizzer clone type sheave made of steel.
MT