How Big can a 5.2 get?

You say the pistons are .018 above deck. I assume you mean all of them.
I reread the thread but I got no smarter; Are those heads closed chambers?
When you measured the deckheights did you measure across the center of the piston and rock it to find the lowest number. If the piston-crown was not dead-parallel to the deck it will skew your deck height measurement, and if you are using math to find the P-H clearance, it will skew that as well.
In any case your range of P-H clearance AKA Quench , is well within the range of acceptability.
But if you find the deck heights changing from front to back or side to side, and the range on one side is different than the range on the other side, then the decks have been machined crooked, unless you also had the block line-honed,and or the rods resized, then there could be some error there.
But if one entire side is about the same from front to back, as is the other side, just different, then you just have one taller deck. So now you can either machine the taller one down to match the other side, or, since the range is .009, you can just even it up by gasket selection.
The thing is, if you also had your rods resized, then they can be the culprits.
What I did was take one assembled rod and piston, and stuff it in each hole marking the deck-height on the deck surface. THis found me the deep and shallow holes.
Next I picked one hole and one piston and and checked the deck height with each of the 8 rods. This found me the long and short rods
All my KB-107 pistons were within .0005 on the cd, so I could put any piston on any rod,except for valve-pocket orientation.
Then I put the longest rod in the deepest holes,with the proper piston, orienting the squirter holes as I went. The rods don't care where they go, except recheck the side clearance when you are done, and maintain some minimum clearance, I forget the spec.
Then I assembled it all, and remeasured the deck heights. I ended up with one tall one and the rest fell within .003 inch, so in my case I was good to go. My deck height ended at .005 +/- .0015, except for the tall one was a thou or something, higher, I forget.
There was a lot of time involved, but I was running out of machine shops. I tell you one thing; if there is a next time, I'm going with custom pistons no matter what the cost; I don't care. By the time I ended up with all that machining, I couldda had money left over with custom pistons.

I hope this gives you some ideas.