So imagine trying to spin the mandrel with it only supported with one 1-1/2" wide bearing surface.
You make it sound like line boring is a rare and dangerous feat. It's done all the time. And there are machines built to do this one operation. Many boring machines utilize some sort tail stock/rest, some do not... all depends on the design.
If you had the money a Rottler F65 is 'less scary looking' than a 'standard line boring machine.
Any well stocked shop should be able to fit you a new main bearing cap with none of the fuss made here. It really comes down to the cost.
Will the cost of the machining be significantly more than the cost to get a another block and have it cleaned and fluxed. I think that $400 would be your cut off point. Another block $100-$200 (hopefully free) cleaned and fluxed $200-300.
