The 50qts are pretty big. If you don't find yourself using the space on your trips, maybe consider downsizing? Or buy a smaller unit for your Titan?
I have a Dometic 35 in my Titan that could easily clear under about any Tonnneu I can think of, commercial or homemade.
My fridge also has a home-made insulated cover, which adds a bit of height/width (similar to a store bought cover). Still would clear fine.
My bed setup consists of a 1/2" ply board bolted to the bottom 2 utili -trac rails. To that I installed a home made slide out for my fridge (plus another for my chuck box).
On top of my fridge I have a metal shelf/table loosely attached to that bedsides' utili-trac rail. The shelf can swing up if I need to access the fridge from inside my Flippac.
All that said, I figure if I didn't have the ply board & slide-out draw, I imagine I'd have around 1-2" clearance from the top of my fridge to the bottom of the bedside utili-trac rails. Likely a bit more.
AFAIK the ARB 35qt & Dometic 35qt I have are the exact same, just different badges. -- my Dometic has a flip up door & the round handles at the end, with led display low power cut-out & quick chill. I mention that only because Dometic tends to change models a bunch from what I've seen. Newer 35's may have different dimensions...