Piston to head clearance

In order to be absolutely certain, I think you should call world's tech line and ask them if they can recommend a thicker gasket. You're right, .026 is too tight, and .039 is where you want to end up. I would not take any material off your pistons, because it will make them weaker. May I ask why the block was decked that much in the first place? Are you using the stock pistons still?

Take it of the pistons not the heads.

I would question the two things here;
1. What has the block really been decked too? Too much will throw the rest of your mating clearances off... and that does matter, so you NEED to know how much has been milled off.

2. If the block is good to go, the pistons may be the incorrect compression height. Or it is correct but milling of the pistons is required.

If you have the part numbers to your bottom end... that should give you the dimensions you need to know in order to do this right.

In most cases, milling a piston has no affect on strength when accomplished by a machinist who does this on a regular basis. It should not be dropped off at the nearest machine shop... it will be screwed up once you get it back in your hands.
On average, custom piston machining is done by a very select few performance builders trying to attain the most perfect compression ratio, cylinder psi and custom head designs with radical cam events.