The $650 and $750 ones from show cars are "off the shelf" Ross forgings.
I have bought several parts from Show Cars, but for the pistons I would order them through someone who can help you get them spec'd right for your needs.
Bore, stroke, rod length, 5/64" or 1/16" width rings, compression distance (deck height), compression ratio, dome volume and height, cam timing and valve relief depths, passenger or truck block, etc., etc. There are a lot of things that need to be correct to suit YOUR engine so I would not pay about the same price for an off the shelf piston as custom ones.
You can get custom ones to your specs for about $750, but then if you have them lightened you will end up at $900 or more. EVERYONE who has built a 348 / 409 highly recommends the lightening.
I paid another $115 or so for coated skirts. I wasn't planning on them, but at the time I was ordering I thought "at this point, what is another $100 or so?" Waited my whole life for this build....and knew I'd end up regretting it if I skipped the coating.
Jack Gibbs, Aubrey, Jim Richmond Engines, Day Automotive, can help you get the correct parts. I bet I am leaving a lot of people out.....my apologies.