I just drill a bunch of holes in the tube for a drop-in lock pin- - - - -maybe every 30 degrees or so. When I'm whittling on the front or back of the block, I bolt a pivot plate to a few cylinder head bolt holes- - - -for machining the flywheel end of the block for a full circle lip seal with my Van Norman boring bar, etc.
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!