Why do I keep burning up A/C clutches

[plutopjf]

I have a 1972 Chevelle with factory A/C. I have burned up 5 A/C compressor clutches and can't figure it out. I have converted the POA valve to a update kit specifically for the 134A. I have installed a Pro6ten compressor (Burned two of them), changed the Expansion Valve. I just don't know that much about the A/C system to know what to look for. I don't really trust my car at any A/C shop. Does anyone have an idea for me?