yes, agreed; a sonic check is definitely in order for this level of overbore. jimmys do seem to share the robust cylinder-wall construction typical of the higher-quaility gm engines of the era, i.e. 320 buicks, 303 oldsmobiles, and the aforementioned 239 pontiacs. 302 gmc's routinely take a 1/8" overbore without sonic check and there are a quite a few currently running, driving examples of 303 oldsmobile builds over on the HAMB that sport a whopping 1/4" overbore and are driven pretty hard. an educated guess tells me that gm probably shared the same core patterns between 248 and 270 blocks and simply bored the 270 a bit larger and machined the decks to different heights. 270's are still routinely overbored to the 3 15/16" size with no ill effect.
