205 and 200 are cossie blocks. 200 are stronger though, thicker walls etc. Any numbered block inbetween will be a pinto block.
I know of cossies running 450 bhp plus with a 205 block (although a n/a would be revvier and possibly more stressful on the block), So provided its in good nick, chuck whatever you want at it!
As far as an overbore goes, I know at least .5 is ok, I think there's some gen in the Burton Catalog iirc