I ended up using some thermal underwear I use for skiing and a synthetic fiber hunting shirt. I had both on hand, so it cost me nothing to try them. The day started colder than I expected, but ended up hotter than I expected.
I started off with a jacket on over everything, but by mid morning I didnt need the jacket and by mid afternoon when it hit the high 50s I was just about too hot. I was mostly happy with everything, but the fiber that the shirt is made of is a little "slippery", so it was difficult to keep it tucked in over the course of the day. And its not like I can easily tuck it back in with the multi belt and the load on. Next time I might try just a cotton shirt over the thermals. But if it had of rained on the day, my set up would have been a lot better than cotton.