Now that I have the head bolts figured out thanks to you guys I can move forward with my rebuild.

Next curious thing is the piston diameter. They all have 91.46 stamped on them.
I measured all 4 with a micrometer (professional one) which I used before and always got accurate readings and they all measure 91.43mm.

What is this? STD pistons for this engine are 91.0mm or 90.9mm (2nd version cc lowered to 2399). I've seen oversize pistons and rings for 91.5mm and 91.7mm but what is this 91.46mm/91.43mm nonsense?

I measured at the thick end of course. The piston gets narrower towards the top to compensate for thermal expansion.

Could this be a custom piston?

I will get a tool and measure the bore diameter soon.