My truck is a 91 D350 dually. To the best of my knowledge it would have the dana 70-u rearend w/ 3.07 gears. Could I swap out 4.10 gears from a 95 F350? I could get the 4.10 rearend for $100. Would 4.10 be too much?

 

You have a Dana 70HD with 3.07 gears. 4.10's would be terrible with no overdrive.

 

What about 3.73's?

 

Where are you going with this?

 

Unless you are planning to swap in a trans that has an overdrive gear, you will be best off with the 3.07's.

 

You can't swap other gears into a 3:07 model 70. The housing is different.