I typically leave the T stat out while filling the radatior. This will push the air to the top and as you watch the water leval rise put the t-stat in just b4 water reaches threads on the housing.

Bad t-stat possable
newly rebuilt? did the head gasket get put on backwards and block a water port?
Leaking head gasket?
bad pump/ loose /damaged impeller-it happens not often though
somebody already mentioned collasped hose and/or plugged radaitor
if it overheats again check for cold spots on rad

Good Luck and let us know what you find
Brad
most likely Air in system take stat out and let set a while to see if leval goes down

http://s320.photobucket.com/albums/nn343/brsdb3381/Tuesday_MY_56_Chevy/

56 3100 6 cyl