I'm using the Vivitar 75-260mm with step up ring 67-72 and the Vid-Atlantic XL clamp. Indeed, inconvenient and extremely heavy to use.
It looks impressive and it is the subject of talks and questions, but as I've been at it for a while now, and I made more mistakes than anyone reading the EOSHD guide ever should (sorry, just stubborn) I now have two EOS cameras with identical 135mm lens and Ishico Proskars and step-down adapters (no clamp required) for two camera shoots. The huge lens was unable to focus close, but with the 135mm and Proskar at 2 meter distance I get an extreme close-up.