its overheating....help

[jared87350]

just put a rebuilt 350 in the truck and it is overheating.
i really dont know what it could be.
it has a new water pump,and i have gone through two diffrent thermos.
belts are tight and has great water flow.the rad. is not new , but in good condition. it has plenty of air flow with a mech. fan and two electric.
ran the engine without a thermo. and it did not overheat. it ran for hours at 160 at idle and 190 at low idle in gear.dont now what the hell is up. Spent a lot of money on this thing and this is pissing me off.
Its an HEI system and the base timing is at 12 advanced.

please help.

jared