I installed a 10+ gallon tank on the inside of my driver’s side frame rail on my '08 F350. It’s immediately behind the transfer case (~1/4" away).
I don’t do any real off roading, but am worried about thermal growth on the transmission / transfer case causing the transfer case to contact the tank.
Any body have a feel / recommendation on how much clearance I should leave?
That should be fine. I honestly don’t think your transfer case will expand 1/4". I might be wrong. If it does expand more, it shouldn’t hurt the tank.
I doubt it can expand that much too.
After a little re-configuring, I finally got just over a 1/4" to allow for any movement.
The tank is 31" long, but almost fit like it belonged there. 
I already put my install details on Diesel Sniper’s web page if anyone is interested.
http://dieselsniper.forumotion.com/leslie-series-horns-f17/tank-mounting-ideas-t91.htm
your install looks very nice.
I don’t think you need to worry about thermal growth as much as you do about movement of the powertrain. These diesels offer lots of horsepower and torque, and some of it is lost in the powertrain due to things like axle wrap and frame twisting (heavy loads).
Nice install bro … I can’t wait to do mine this summer.
Kris