I think it's great what you're doing with this model but just need to backup something mentioned earlier regarding the refueling valves.
I've lost track of all the engines, including my own that we traced erratic performance down to those valves. Nothing but trouble.
They're sure neat and dandy... but actually aint worth the bother. Build them out now whilst you have the opportunity because you'll be bypassing them at a later stage. Garanteed.