Plastic intake still?

If yes then at least 11 to 1. The 6.4 is about there with 205 thermostat and a lean emissions friendly mix and you know they put a little room to spare in it. Probably be silly to go under 11.5, these things are very octane tolerant.

Aluminum intakes heat the air a bunch and will probably take 1 to 1/2 point less compression.