I'm going with the route of boring the monobloc on a mill. The tubes have been cut to 3", and the bloc has been faced.

The school has boring equipment of course, but I was wondering, for this use case, what do people like to use for boring this length of tube vertically?

Also, what is best approach to hold the block securely, considering the taper of the remaining tubes, and keeping the boring straight.

Tubes were found at Holt's: 28 1/8" unchambered Westley Richards tubes.