Congrats on the engine.
re. thermostat. It's an old bellows type thermostat from back in the day. I've been running the one that came in my 216 for 6 years without any problems. It is a 160. I tried a 180 for a short time and went back to the 160. The 49 3100 was parked in 69 so that thermostat I'm still running is atleast 50 yrs old...... and still works......
Good luck.
Pic
Pic 2

1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise