first build, Chicago, oven floor questions

[dancfd]

Hello, into my 1st build 36"dia, prepareing to set the firebrick floor and am having trouble locating 2 1/2" fire bricks. Is it acceptable to double up 1 1/2" fire bricks for the floor? Making it a 2 layer 3" floor? how far should the chimney vent be from the outside diameter of the oven? do i need to add extra insulation between the oven and the walls of a gabled oven build? thank you