There where 2 different piston heights, 1.640 & 1.655, due to the water pump bolt as tight as it is to the top of the deck and a known crack issue here I wouldn't cut more than .025 off of the deck on a 250, also as stated befor maybe the block may have been a slight bit taller, in this case I would check the piston to see what the compression height is first, if its the 1.655 you might then be able to deck the block even more, I would also check another block and measure the bolt hole to the deck on both blocks.
