Steinke, you have a lot of room.. and you won't have to use a jack shaft.. I'd buy a 8" pulley.or larger, it looks like you have room for larger... from 6" to 8" will be 33% decrease in the stoker shaft speed. I think my math is correct..
A jackshaft is a separately supported shaft with in your case, probably a 4" pulley driven by your motor, and a 2" pulley that then drives the stoker pulley,, this would give you a 1:2 reduction in the stoker rpm. with full speed on the motor and fan. You would need to make a bracket to support two pillow-blocks for the jackshaft to run in, and some slotted mounts for belt tensioning..
But I'd try the larger stoker gearbox pulley first, it is the easiest and quickest to try.