A forged 360 Mag piston that comes in around 8- 8.5:1?

The lowest compression forged pistons that you'll find without going custom or remachining are NHRA/IHRA "Stock" class pistons. However, new, these tend to be in the "custom" price range. They do have a lower compression height similar to OEM to meet the rules. They are forged and I've seen them advertised to be ok for nitrous up to 250hp but not turbos. Not sure how a mild supercharger application would fit in. Check with the company or your engine builder.

Here is one example that you can plug the specs into your compression formula for comparison.
Consider high quality, thicker head gaskets also and milling the deck or heads an absolute minimal amount. OEM decks tend to be a bit taller than advertised..

Low compression 360 piston ad on this site.