Personally, even two inches of distance is a bit too tight for a good flame distribution. Burners have hot spots and for a better heat distribution, more distance would be better. Four inches would be plenty enough for fitting in a main fuel vaporizer, and also a super heater in properly. Any more than four inches of distance would be wasted head room. More distance would still work as well though but outside suspension clearance problems would then begin to occur. Presently the tightest distance that I have done is about 3.25 inches between the top of the burner grate and the bottom of the tube sheet. That distance is with a 23 inch Baker Burner installed under our 23 inch Stanley fire tube boiler.
Bill Iíve built them with as much six inches.
The heat does not know how far away it is, itís like the sun it radiates in all directions at the same speed as light. There are hot spots as well, like the tip of the blue flame just like on your cutting torch.
Much to hot for the vaporizer.
I put the vaporizer as high as I can off the burner, and keep it as short as I can and still get clear gas.
Like Pat said itís clearance space under the boiler. If your in back of the front end I donít like to go below the tie rod. Over the front end it depends on how good the front springs are.
Reading a Stanley service bulletin for the Condensing Stanleys, and the distance listed from the burner plate to the bottom tube sheet was 5 inches. Just finished a 1909 20 HP 23" Baker burner and I used four inches distance between the two. It had plenty of room and it looked like it would be happy in operation.