2009 supreme 1 rear spring weight

Just got my supreme 1 from chainlove on Friday, Put it together and immediately loaded it for the race at beech mtn, NC, the bike felt great given the lackluster fork and I done almost no tuning aside from letting all the air preload from the fork and dropping the boost valve to ~75psi on the shock.

As a 120lb rider, what spring should I be running on the rear? The stock 400 feels like it sags a lot but feels rather harsh in the chatter and rock gardens.

Also has anyone put a dual crown fork on one of these? Considering throwing an older 888 on it to see how it feels.