you can't tell if the block needs work until you remove the head
if the corrosion is bad and has damaged the block you would need to resurface the block.
there isn't much point to using a metal head gasket on a non turbo engine .
the whole point of perfectly smooth and a using the metal head gasket is to handle the extreme boost planned for a seriously modded turbo supra
i know all about lapping the block
been there ,done that , got the T shirt
http://www.ddmcse.com/turbo/lapstone_small.JPG