Oh, Mr Jarse, you're asking for trouble with a question like that!
It sounds like a pretty major mod. Right behind & below the seats you've the fuel tank/s, you've the bulkhead that seperates the cab from the load bay which I'd think means messing around with the tub which is no fun, can't see the point in bigger doors unless you move the seats back otherwise they're going to be in the way.....
Jeesh, the more I think about it, the more I'd say it wasn't feasible. It'd be a shed load of work, you'd end up with a fairly ugly III by my reckoning & if it was my huge arse I was trying to accomodate I'd think about buying another product from Solihull's finest. Though it pains me to say it (sorry guys!), have you thought about a Disco?
Er, sorry, that is a serious answer!
Best,
Marcus.