120 is a lot. On my beater Ford I have a tire that likes to go flat overnight, somedays I just give it all the air, so I'm not waking up to a flat the next morning. Never hit it with a gauge, but sometimes I can definitely tell from the roughness there is a fair bit in it.
I just recently mounted some new rubber on the dually. Which is a lot of damn work, I will say. Anyhow the old tires I was running, and these new ones. I just put 50lbs in. They are 24.5's. Bought some with open shoulders, so they look more like a traditional truck tire.
People will probably say you cant do that, but so far Ive had no issues. Maybe try airing down a little if you're not going to be pulling anything. I pulled a few people out of the snow last winter, never had any slippage issues on the worn out semi truck take offs, with dry rotted cracked beads. Though I don't really plan to pull much, and its relatively lightweight for a 3500 4x4 dually.
I have no clue what numbers are on my torsion bars, but I threw the ones from a 06 2500HD in there, along with 06 2500HD lower arms. 50lbs in the tires, and maybe the new big RC shocks help also, but it rides really nice with the keys pretty much cranked.
"Really nice" may be subjective. Ive had 4x4 squarebodies that didnt ride much different than an old 64 C60 grain truck, but this rides nice to me. (Aside from current new tire rubbage issues.)