Most of the original engines didn't last long without getting the poor-design pistons replaced. Chances are it's been rebuilt at least once unless it was taken out of a low-mileage wreck. Any small block Chevy with much over 50K miles will have enough taper wear in the cylinders to need a rebore job. The good news about the pre-65 blocks is that they can be bored to .060" oversize without a problem. The max for later engines should be .030" due to the thinwall castings they use. The early 283's can even be bored .125" over if you're willing to go to a heavy-duty radiator. That's where the early "302" Z-28 Camaros came from. Later models used a 350 block with a big main bearing 3" stroke crankshaft.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!