Howdy,
What I found last year whas that if your grate spacing is too wide, your coals will drop through to the ash pan and it can get very warm. I ended up cutting down on my grate spacing, but I also added insulating firebrick.
Check out this web site. http://www.sheffield-pottery.com/SOFT-B ... -s/372.htm
I actually cut the bricks in half longways to line the bottom of my ashpan, and used the full thickness on the sides. It will make sure you dont have an issue. I would still leave an airspace between the boiler and the deck. I went one step further and put a sheet of aluminum diamond plate about 1/2 inch above the deck and the bottom of the boiler is about 1/2 inch above that.
These guys cut real easy on a table saw. Use a regular cheap blade and toss it when you are done. You dont need a masonry blade and they are stinkin expensive anyway.
Of course, you have to have room in your ash pan to do this.........mine is quite deep.
jon