How much off the block?
The machine shop shouldn't be asking you how much they should take off. They should be asking you where you want the pistons to sit in relation to the deck. Then they should be measuring the deck height (they are more often than not higher than the nominal 9.600") and squareness and tell you what needs to be done.
And to determine where you want the pistons to sit you need to know what you want for a compression ratio and to do that you will need to know the actual chamber volume of your heads as well as the piston volume (dished, flat top, valve reliefs, etc) and the head gasket you plan to use (compressed thickness and bore). J heads could be anywhere from 68 to 72 cc, so you will need to meausure.
One of the online compression ratio calculators will help, like this one http://www.kb-silvolite.com/calc.php.
BTW, if they are using a machine like this with the same type of fixturing and they remove material from one end to the other the deck will be square.
The machine shop shouldn't be asking you how much they should take off. They should be asking you where you want the pistons to sit in relation to the deck. Then they should be measuring the deck height (they are more often than not higher than the nominal 9.600") and squareness and tell you what needs to be done.
And to determine where you want the pistons to sit you need to know what you want for a compression ratio and to do that you will need to know the actual chamber volume of your heads as well as the piston volume (dished, flat top, valve reliefs, etc) and the head gasket you plan to use (compressed thickness and bore). J heads could be anywhere from 68 to 72 cc, so you will need to meausure.
One of the online compression ratio calculators will help, like this one http://www.kb-silvolite.com/calc.php.
BTW, if they are using a machine like this with the same type of fixturing and they remove material from one end to the other the deck will be square.