Here's a gear calculator
http://www.onlineconversion.com/bigger_tires.htm

This is not a silver engine, and is governed at 2,800 rpm (silver 53T engines are often governed at 2,500). However 2,400 is plenty fast enough for this one and all day driving, and 1,900 or there abouts will be more ideal for this truck when empty.

1,900 rpm with 38" tires and 3.31 gears gives 65 MPH, good for empty, on second thought might not be the ratio we want.
Or if the truck is often loaded a more reasonable ratio might be 3.54, that gives 65 MPH at 2,040.
Or figuring the current 3.73 ratio 65 mph is 2,150, which is close enough to run it for a while and then decide if it works or if it needs a faster gear or two.

Just for fun top speed of 85 MPH at 2,800 with 3.73 gears, or could hold 75 mph at 2,500 all day if you like to buy fuel.
3.31 gears bump those speeds up to 95 and 85 mph.

Nice thing about the Dana 80 is it has plenty of choices even up to 3.31, which will help in some cases when OD is not available, and has one faster gear available than a Dana 70 or 70HD.

Grigg