Dave,
If you can, order a 24 x 48 .....
A 36 is way too short and a 42 add only six inches.
You WILL lose real estate to the firebox side, that is just a fact ...
Also ... depending on how 'serious' you are .... the firebox lid is up for a strong debate - to include or not include.
No matter what anyone will tell you it will cook very differently without it vs. with it ......
I owned a Klose ... same style pit all the way around ... had the lid ... and could never control my temperature.
Many kept saying 'you have leaks, you have uncontrolled air' ... I thought, no I do not.
Finally, I had someone weld the FB lid shut ... .. removed the handle, removed the hinges, put two nice beads all the way around nicely. Welded Shut.
The smoker was an entirely different smoker.
Since then, I've heard versions of the same story over and over ....