Well, according to some analysis done in this thread:
http://classicalpontiac.com/qa7/disp...-+why+no+350?&
All of the pontiac blocks appear to be the same casting, so it should be possible provided there isn't any shift moving the water jackets closer to one side of the bore or the other. I think it needs some more in depth analysis, but per above the 350 casting should provide a wall thickness of .207" at 4.181" bore (455 + .060" overbore) which is what the recommended minimum wall thickness is, and is probably very comparable to what an .060" 455 or .090" 400 has...
Somebody should really take one of the thousands of 'scrap' 350's (or even 326's) around and try it or at least cut it up so we know for sure, it seems like everyone just assumed it couldn't be done. In the 7 years I've been around Pontiacs I've never heard of somebody trying it, only that it couldn't be done.