Okay, I'm now (happily) almost $1,000 poorer
I've just ordered from RRS the:
BH-40 Pro II (with screw clamp)
a base plate for the 5D3 (with BG)
the L-plate (for the same combo)
a base plate for the 300mm f4.
I've also ordered a QR plate (with QR clamp) to use on the Manfrotto 496RC2 I have, as I have that on a Gitzo monopod. After some discussion with folks at RRS and using their compatibility chart, I reasoned that the one that fits the previous model 486RC2 may fit the newer model. If not, oh well.
My aim is to head down the Arca-Swiss style on all my gear. Over the years, I've built up an odd collection of tripods, heads, etc. Time for me to clear out that collection
Thanks for the informative feedback, not just this thread, but in others that have recently covered tripods and such.