I'm not sure how much you've thought about this, but in all the options I've read, you may want to consider the following:

Modified clutch disk - when replacing your clutch later, you'll have to either rebuild your current disk, or buy a new one and modify

Thicker flywheel - you'll be losing a lot of power by adding more weight to the crank. A couple pounds here equals a lot of power loss further down the line (ALTHOUGH, if you need a thicker one, you might be able find a lightweight one through an aftermarket suppplier, depending on original application)

Modified flywheel - if you crack it, you have to find another one, and modify it the same


For me, I'd look most at either grinding the bellhousing on each end or have that little adapter made to fit between the flywheel and crank. For the adapter, you may want to have to made so that the bolt holes are countersunk, and use short enough bolts so that you can bolt the adapter to the crank WITHOUT the flywheel and the bolts won't protrude from the top. Then, tap holes in the blanks between crank/adapter bolts and bolt the flywheel to the adapter. Of course this rules out aluminum and I'm not sure if that came out right, I'll try to draw it or something if not.


Still thinking...