my cab was off a 4x4 hence the huge square hole filler piece. and the filler is actually overlapping bothe sides of the cut bout 1/4"
and you dont have to z it to get it to lay flat you can cut down teh part of the frame right under the firewall and move the mounts down pretty much flush witheh top of the frame... mines 2x4 1/4 wall set inside the stock frame with whatever sticking over the top cut off, if i remem right was like 2 3/4" or thereabouts.