283 V8 looses oil pressur at driving temperature

[oleguy]

I am about to acquire a 283 out of a 66 chevy pickup.To put in my 67 C10. It was recently (overhauled) by a previous owner. The oil pressure is good at low to medium temperature, but at running temp. it drops to near zero. My thoughts are #1. Main bearings to much clearance. #2. Cam bearings to much clearance. #3. I don't know if the oil pump was replaced or even checked. I will tear the engine down and check these items. Any further ideas on the the cause of this problem will be appreciated. Thanks in advance.