Lean under load

I'm running lean under load (going up a hill at low RPM), what do I need to change to richen it up a bit? I have installed a A/F gauge and here are the readings i'm getting:

- Idle is around 13 - 13.5
- Part Throttle (Highway driving) 13 - 13.5
- Full Throttle 10.5 - 11.5 (so too rich, should probably lean out the secondaries a bit)
- Part Throttle under load: 15.5 - 16 (going up a slight hill in top gear at low RPM)

Specs:
- A/F Sensor installed just behind the passenger side manifold flange
- 650 Edelbrock Thunder AVS
- '74 360 with electronic ignition, 18 initial and 36 total timing.