A 2x6 flat pan will not handle 400 taps, it should boil away about 12-14 GPH if the arch is designed properly and you have good dry wood, with good draft. Then just figure how many hours you have to boil. As stated above, even with 3/16 gravity vacuum you can get 2 gal/tap/day on good days. Do the math.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.