Since the shop building his engine has already told him to get .010" over pistons, they should have already verified that .010" over would clean it up. That sure isn't much, but I've seen old blocks in really good shape.
As hard as it is to find a good 340 block and as thin as their walls are, I'd go the absolute minimum it would take to clean it up.
CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax
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