yup thats exactly what i'm say. set screws holding anything together is a *really* bad idea.
do it like rupp did it. they treaded the end of the jackshaft 5/8-18 so a lock nut could be used to hold the jackshaft parts in place. Or drill it and tap the end of the jackshaft at 5/16-24 and put a bolt/wash at the end. do *not* use set screws for *anything*. get 5/8" ID stainless tubing as spacers for sprockets (if needed), and throw out the set screws.
set screws are for rookies. *especially* with a 30 series torque converter that pulls outward, and a high horsepower predator engine. it *will* come apart with set screws, that's 100% for sure. and it always happens at the worse times...
it's happened to me twice with set screws. once i was 2 miles through woods when the sprocket set screws came off, and the keyway was gone. that was a really long walk. the second time i was on a Harrison at Miland dragway (michigan) when the set screws came off on a 1/4 mile race, leaving the engine at no-resistance WFO, and threw a rod. i will *never* *ever* use set screws for anything. hard spacers and end bolts on the jackshaft is the right way to go.