Yes , I use the one piece white metal floorpan which I buy from Brian Norman .
Its a bit of a fiddle but you basically cut out most of the centre section which weakens it a lot.
I then file down the centre of the 6600 casting flat (where the gear levers would be) .
Then you can have the flat floor in the cab .
I usually add a couple of pieces of wire to look like gear levers ,which if you drill them through the floorpan into the 6600 casting underneath ,also help to hold it all in place.
I hope this all makes sense ,its hard to describe how to do it really.
I just bought the bits and had a bash .
As you know though,white metal can be a bit flimsy to work with ,so take care .