.003" runout on a flywheel is huge- - - - -get ready for lots of clutch chatter. I hope you used a tool post grinder to get the right surface finish, also. There are complete overhaul kits for the SM 420 on Ebay with bearings, gaskets, and other small parts, along with plenty of gears, synchronizers, and other hard parts. Those transmissions are so bulletproof that rock crawlers install them in virtually everything. If you're having clearance problems with the crossmember while installing the clutch, unbolt the rear motor mounts and jack up the bellhousing a little, and/or install mounts that aren't worn out and collapsed. I've installed dozens of those clutches with no problems whatsoever.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!