Hi, I've got a Kirk convertor that simply slots into the RC2 plate and converts it to Arca, without changing the head. It's an extra. I've also got a Kirk L plate for a 5D2. Always worked perfectly for me. The convertors are rarer than hen's teeth - perhaps because everyone is trying to persuade you to buy a complete new system. Even Wex - the UK importers for Kirk - don't stock them!
They are now both surplus to requirements, and I'm happy to let them go, but don't for God's sake ask on this thread. If you're interested in them just start a wanted thread and we'll talk there.