There isn't much of a gap between the pan and the top rail when the arch is cold, but once it's been running good for a while you'll notice that gap getting larger. Look straight front to back along the side of the arch and you'll see the sides are bulging out from the heat. This year I added about a half an inch of rock wool behind the fire brick and the bulging has decreased a lot. I think the better solution would be a quarter inch thick ceramic blanket behind the fire brick.
2016-First year
17 Taps
Block arch & steam pans
About 1 1/2 gallons
2017-20 taps
About 3 gallons
2018-13 taps
Same Block arch & steam pans (modified arch & pans)
2019-12 taps (keep losing trees)
2020-Brand new Smokey Lake StarCat evaporator, about 15 taps.
43.62N 87.78W