Let me ease your mind. My first F 350 SRW crew cab 4x4 was in 1996. It had 355 gears and when I installed 35" tires it would not get out of its own way. Had to trade the tires in for 33's. At that time stock tires were 31"I am special ordering an F350 6.7L 8' bed 4x4 single rear wheel for towing a 35' 15k lb toy hauler.
I am shocked to see that you must switch to dual rear wheel to get gears lower than 3.55.
I plan to only install a leveling kit and 34 to 35" Toyos on either 18 or 20" wheels, but 3.55 seems light.
I would be comfortable with 3.73, but had planned to push for 4.10.
Our trucks have a factory 20" wheel option with the stock suspension set up (no leveling kit required). Going with after market 20" wheels shouldn't be a problem height wise, but if you go with an excessively wide wheel/tire then it may start creating problems.When you configure the truck with 20" wheels, it changes the front suspension.