At the back of the firebox area on those smaller arches it is more common to build a wall out of firebrick. Have the top of the wall close to the pan bottom and level from side to side. To calculate the space figure the cross sectional area of the stack in sq. inches, then divide that by the width. The result is what space you want above the wall. Behind the wall there is then open space to let the smoke drop down to exit the stack. Some fill that space with anything to just fill the void, from fire bricks to vermiculite to sand. The issue with sand is that it tries to push the wall forward. For a 34-36" long arch the firebox might be 20-22", then 4.5" wide firebricks, leaving 7.5- 9.5" behind the wall if you go out the back. If you go out the top with the curve of the original tank top as a base stack you will have more space to fill, but it will be a more efficient design.
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.