No need for a ballhead on a monopod, IMO. I'd go with a tilt head. Definitely agree that an Arca-Swiss compatible clamp is best For the load you're considering, I'd get the Manfrotto 234 tilt head (just 234, not 234RC which has a non-removable proprietary clamp).
I just looked, and the 234 seems to have been discontinued. Adorama still has them, as do some 3rd party sellers on Amazon. It's a great head, act fast! I know you mention wanting 6-7 lbs vs the spec'd 5.5 lbs load, but I used a 234 for a couple of years with a gripped 7D and 100-400L (I only replaced it after getting a 600 II).
For the clamp, the 234 has a reversible 1/4"-20 3/8"-16 stud, so you can attach a variety of AS clamps. I used a Wimberley C-12 on my 234 head, but the RRS B2 LR II lever clamp or B2 Pro screw knob clamp would work, as well (as would similar clamps from Kirk).