Sounds like a road race to me on such a big track. I would start right around 30 pounds or maybe less if you don't weigh much. Lower 'supermoto' pressure levels are going to overheat your tires and they will wear out pretty fast at the higher speeds you will see on the bigger track.
I usually ask around and see what the other folks are running. Maybe find someone running a TZ250 and see what pressures they are running then adjust from there over practice to get things just right.
Gearing is your other problem which you will find plenty of advice already listed in the forums. Search for '450 track gearing' or similar.