I think you'd be better off making it 4x18 now. The 6" stack (or later an 8" ) stack is up a few feet, typically minimum 3 up to 6' above the base opening. The transition then allows the exhaust flow to concentrate to the pipe diameter. As small as a 2x18 gives you the sq. in. needed but it still chokes off the flow. In fact I had my base opening on my 3x8' rig made 32" wide, you will get better results if you widen it to 20 or even better 21" wide. That will help you get the best heat distribution near the back of the pan.
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.