Deck height

you can make a piston stop from a strip of steel, long enough to bridge across one bore.

drill holes in the strip that align to two head bolt holes, get shorter bolts to bolt the strip across #1 bore. It needs to be flush down to the block deck.

drill a hole in the center of the strap that will accept a bolt and nut. Position the bolt and lock with a nut so the stop protrudes maybe 1/2 inch down in the bore. The depth into the bore needs to exceed the piston recession being measured, but where to set it is not critical, put it where you get a workable range on the degree wheel.

the strap needs to be rigid enough so it does not deflect when stopping the piston.. I use a piece of boxed channel.

you can print a degree wheel off the internet,, or buy one at Summitt, Jeggs, AZ,, bigger diameter has better resolution, less error.

remember, when you get #1 at TDC,, after measuring it you can go right to #6 and measure it as 1 & 6 are both at TDC at the same time, that will get you machining run front to rear on the block. Piston crowns can have a thou or three run across the top, so to get repeatability measure approximately the same spots on the pistons..I measure about 1/2 inch off piston center.