As mentioned above, make sure you follow the procedure to "burp" the system. You can remove the thermostat in about 2 minutes if you suspect that's the problem. Also look at the coolant tubes under the rocker panels. It's pretty common for someone to run over a curb and smash them flat or kink them. Make sure there's no kinks in any of the hoses.
------------------
White '85 4cyl coupe (daily driver)
Black '86 SE 3.4 V-6 auto (Project #1)
White '88 GT 5-spd (Project #2)
'81 CB900F Super Sport (Completed Project)