In the combos I have been lucky enough to be able to study, a 7.100 big block Chevy rod is usually used in both.

If you wanted to run a regular shelf BBC rod (meaning not custom length), then 540 pistons will have a taller compression height
than 572 parts. Along the lines of street engine use, this will give you a taller and more stable piston which has the side effect of being
slightly heavier, nothing to worry about.

A person could get around this if connecting rods longer than 7.100 were readily available on the shelf, but alas that isn't so.

On the other hand a 572 with 7.100 rods is well within the known safe zone for a street engine, as it has the same rod to stroke ratio as a Pontiac 455,
and is still slightly higher than a Chevy 454.

Indy (diamond) is the path of least resistance for 572 shelf pistons, they usually have them on hand for crate engines. And maybe they are nicer to deal with now!
They may not be perfect in every way, or have the latest awesome tiny metric rings.

Just wondered why and am not trying to be persuasive....

can't help on the whole 6 pack thing...I have a brand new setup collecting dust but not sure if I will ever use it. I keep collecting
parts for an eventual project, but who knows if it will ever happen.

Best of luck in any case