If the power is never going over the 500 hp mark i dont think 1mm over size is a prob but what you have to remember is the block is on its last life then so if any trouble or when it wears out it will then need linering?
it all depends on what needs doing to it to get it where you want it,ie if it needs to be machined for new pistons long studding etc then i wouldnt use a 1mm oversize block to begin with..
Any other bore size is nothing to worry about at all..
cheers danny