Either way, you have a lot of work... Realistically the height is a no go...You ideally don't want to be much above 3.5m total height. I also think that if you have rust to repair in the red cab as well as cutting the floor out and trying to make that work, the work involved is overtaking the gains to be had with the red cab... The other way of looking at it is that you are building a camper with a huge living space, so do you really need a sleeper cab? I originally wanted a bigger cab, but now I have the day cab it looks like it will work well with our 6m body. I'd be going for the easiest option. Also, don't forget, there's a lot to be said for keeping stuff standard... (says he who is making everything body related). With a standard cab chassis, parts will be available.