John
Do some sea trials with and without one prop at speeds of 5-8kn. You might be surprised how little difference it will make to remove it.
I have attached a chart I posted recently on another thread. For both Dreamer and my boat, running on both engines gives slightly better mileage than running on just one engine, at any given speed with one prop freewheeling. Part of the reason is that I needed 9 deg of rudder to go in a straight line using just one engine. To travel at just 5kn I would need to run on one engine to keep the engine at a warm enough operating temperature, but otherwise would not do it.
Dreamer's data is here.
Dreamer
A GB 49 is about the same LWL as my boat. I'm 65,000# at full load whereas I think the GB will be quite a bit more at full load. My boat has claimed lightship of 48,000# whereas the GB 49 is 60,000# lightship. I believe I was actually 55,000# lightship at refit, and expect a GB to also be a be around 10% heavier than when delivered bare. So you might not get as good NMPG as I can. Of course the Lehman's are renowned as fuel-sippers, so some sea trials at the outset will be prudent.