True, the exhaust does resonate at certain engine speeds. Hopefully the wire hangers I'm using to brace it in the middle will minimize that effect.

The BuKu clutch really is an ideal setup for this engine. I can use the adjustment screws to preload the clutch springs as much as necessary to keep the heavy shoes from engaging too early, which means I can use normal (or even soft) springs to ensure the shoes engage fully as quickly as possible once the engine revs-up to the engagement RPM. Stiff springs would keep the heavy shoes from engaging too early, but also prevent the shoes from fully engaging as quickly as possible, because stiff springs require a higher amount of force per each degree of bending. With stiff springs there's a wider range of RPMs where the clutch shoes are only partially engaged.

I'd appreciate a picture of the fan bracket you're using. Is it wire, sheetmetal, machined billet, or something else? I'm just trying to figure out how much effort I'll need to put into making a fan bracket that won't break, rather than figuring it out through trial-and-error.