Do you really think you'll be machining all the way through pieces of metal that are more than 5" thick? Most things are going to be a lot thinner than that - what are you planning to make? The knee will go down to accommodate larger pieces; you just won't be able to machine all the way through them - but the tools won't be long enough for that anyway. I'd say go ahead with your conversion, and be able to make the 99% or so of the parts you'll actually be dealing with. Leave the monsters to the monster machine you build next.