Warping the entire block is not that likely. However, it is easy enough to warp a local area in the block, because localalized hot spots will occur when an engine is overheated. Typically cylinder bores will warp, especially in thin wall castings of a Chev 350. A Chev 350 cylinder walls will range between 0.075" to 0.090" thick due to the draft required to pull the core during casting. If the engine is bored 0.030" over, the wall thickness can drop to 0.060". For all practical purposes, this is considered the limit for ductile cast iron. I have known a few people who have tried going 0.040" or 0.050" over on a 350, but the engine only lasts for a few thousand miles before the cylinder walls warp enough to cause a drastic drop in compression. It is also possible to warp bearing surfaces (Such as a couple of certain V-Twin's at work right now, but that is another post) if the heat is localized in that area. Flatheads are notorius for warping the deck and area round exhaust valves to the point of cracking, since that becomes a hot spot.
I apologize for the long, rambling answer. (I can't help myself)
