My experience over the years towing three different size fifth wheels with 3 ford trucks has been as follows:
32' or less, ok with 3.73 rear end ration
32' - 35' - go for a 4.10 ratio
35' - 38 - shoot for a 4:30 ratio
38' and over- buy a class 8 and single it out.
Lots of variables (I know) especially weights as the higher end fifth wheels tend to weigh more. I presently have an F450 with a 4:30 ratio (6.0 diesel) and average around 10.4 on flat land here in Fl. at 63 mph. I am towing a 35' HR. I towed the same 35' HR with an F350 w/4.10 ratio and it towed fine. However, I felt on the edge many times with the brakes, etc on the f350 DRW. With the f450, never feel uncomfortable.