I am restoring a 69 z28 camaro. the build date of the car is 2 week of oct. the engine has been decked and i tried acid to bring the empression back but won't work. the vin # is stamped down by the oil filter housing. the casting # on the block is 3970010 with the date code being a M code. with some research theres no way to determine the date with a M code block. Doe's anyone have any suggestion how i could bring the empression back to where it has been decked and removed? Thanks Mike