Look at your ratchet lid tranny and compare it to the cowpie...look at it from the mainshaft side. See the how the round pawl carrier "ratchet" extends a bit beyond the side of the actual tranny case? Notice how at "4 o'clock or so it limits the amount of space to the mainshaft. The cowpie doesn't extend out like that so you can run a larger diameter sprocket on it when compared to the ratchet. And that more limited clearance of the ratchet lid is the problem...belt drive pulley is too large in diameter to clear.
To be honest, I love the rear belt on my rubber mount Geezer Glide...but on my 4 speed Shovel I love my nice and simple o-ring chain. A whole lot cheaper....almost as maintenance free when properly installed...and certainly a LOT easier to replace in case of emergency. Not to mention...you have a lot more gearing options too...more sprocket size choices than any belt pulleys.
I will also take a ratchet lid over a cowpie any day of the week if given the choice. It not only performs better...it LOOKS better too.
By the way...clutch cable is fine.
Good luck!
Just a few things to ponder.
