It sounds like you'd be better off to use a different base engine. .030" undersize (not over) is as far as most people are willing to regrind a crankshaft, and .010 or .020 is MUCH better. However, that has nothing at all to do with the size of the bearing bores- - - -they're always the same size, regardless of what the inside diameter of the bearing is, and all of them MUST be bored in a straight line. The oddball thing about old Chevy sixes is that each main bearing is a different size- - - -smaller at the front and they get larger as they go toward the rear.
I have block and crankshaft size specifications if you need them- - - -I can copy and email them to you if you need some dimensions to work with.
Jerry
