Personally, I wouldnt run more than a zero deck. But any time you do this, one side of the piston may be sticking out say, .005, the other side .005 below deck, and the middle at zero. The squaring of decks at the factory on these older pushrod motors is horrendous, because of the old worn out gang tooling they used. I used to see blocks that had a deck that was crooked by .020" on the short distance, .040-.050" on the long distance, and out of square by quite a bit....also they would have 2 different deck heights. But this is a worst case scenario. Most of the blocks machined in the mid 80's and later arent that bad, but I still did see some that were pretty F'd up. If you want an accurate piston depth, make sure to find a shop that has a squaring fixture, and have them use this to zero deck your block. It has a bar that goes through the mains, and there are 2 plates connected to this bar. The whole thing is tied together by a long bar of all thread that runs through the cam journals. Then 2 threaded cones screw into the 2 end cam journals. The whole block/fixture assembly is put into a decking machine, and those 2 plates get zerod in, (the 2 angles on those plates are a perfect 90 degrees apart..the plates simulate a perfect square deck) instead of the deck being used as a reference. You have to consider rod stretch and a few other minor things, so having the piston out of the hole isnt a great idea. Before a block gets bored, there is a conical shaped tool with sandpaper on it, thats used to grind a quick chamfer around the top of the bore. Make sure the shop you use, keeps this chamfer to an absolute minimum, or the quench achieved by all these tricks will be thrown in the garbage. They can be a little more aggresive in the areas around the valves (for unshrouding), but other than that, keep the chamfer very small. My old work, JMS racing engines in monrovia, ca. has one. My buddy told me that JMS was on the history channel a couple weeks ago, on a history of hot rodding episode. I cant believe how famous they're getting now. They're used by all the hot rod magazines out there now too.
