.060"-.080" is considered to be the maximum safe overbore, but these old engines have a lot of iron in them. I've seen race engines go 1/8" (.125") over and last for quite some time, but I wouldn't recommend that for street use. When installing thinwall cylinder sleeves we bore 3/16" oversize (0.1875") without penetrating through to the water passages, and a 1/8" wall sleeve needs a 1/4" overbore. It would be foolish to bore one that big without installing a sleeve to bring it back to standard bore, but the metal is there to allow holes that big to be cut.
If the other cylinders aren't badly tapered, it might be possible to sleeve the pitted cylinders, hone the rest, and re-use the pistons you have now. There would need to be less than .005" of taper wear, however. If you choose to do the full rebore, have the machinist bore the rust-pitted cylinders first, and whatever oversize it takes to clean them up will dictate the oversize the rest of the cylinders need to go. Be sure to replace the cam bearings, and clean out ALL the oil passages during the block cleanup. I use rifle brushes and long gun cleaning rods, after removing all the oil gallery plugs, followed by a thorough pressure wash of the oil galleries.
Jerry
