If you take the front cover off, there's a series of numbers stamped into the front of the block with a dot matrix metal scribe.
There's a set of those numbers that can tell you what pistons are in that engine without taking the heads off.
I think GM posted a pic of some of them a while back; it will be a series between 1 and 3, 1 being smallest bore, 3 being biggest.
My mark engine was a 22223222, with one piston 3.572, the others 3.552, IIRC. It weighs more too.
EDIT: Actual sizes of factory pistons, not my guess above. There is a 1.5g difference in a 2 and a 3.
Piston · Diameter g 0.028 mm (0.0010 inch)
· Coded Red 1 90.185-90.205 mm (3.55-3.551 inch)
· Coded Blue 2 90.195-90.215 mm (3.55-3.551 inch)
· Coded Yellow 3 90.205-90.225 mm (3.551-3.552 inch)
Coated Piston-to-Bore Clearance -0.005-0.025 mm (-0.002/+0.001 inch)
Likewise, there are other numbers that indicate bearing sizes, but IDK how to read those.
You want to buy pistons, even if they're new HE pistons, and not forged; the engine will work much better if you bore it to even size, match the pistons in weight, and balance the crank.
These engines are pretty darn expensive to rebuild to miss a step, especially if you want to use nitrous.
I personally would not rebuild a stock iron 4.6 block; unless it was for a truck.
There are tons of aluminum alternatives, early and late.
80lbs in the front is a big difference, all around.
EDIT: Was the intake already off that engine, in the JY? That reeeeeaally looks rough...