Changing rear gears in my 70 dually

[w201028]

Hey guys, I'll admit I did not do a lot of looking around here for a guide so please forgive me if this has come up a million times in the past. I would like to change out the 4.10 gears in my 1970 1 ton dually for something more highway friendly. I don't drive the truck to work at all anymore because its screaming going down the highway, and I feel bad treating my rebuilt motor like a high rpm generator.
I buy almost all my parts from rockauto and they have 3.54 gears and an install kit which claims to include shims, marking compound, crush sleeve and more that the instructions claim is needed. I read through the instructions but there are no clear pictures of the process and what seems like a lot of steps involving checking the mesh. I'm no rocket scientist but I am a good machinist and have successfully rebuilt many motors and transmissions, including the 360 and t-18 in my truck.
I've replaced the axle seals on the truck in the past, and I have some work to do with the wheels off anyway, so I may as well do all this at once and change the gears while I have it apart.
I'm almost certain its a dana 70, a friend and I looked up the number on the diff before, and I will definitely check it before I order parts. I think the weight was some astronomical number like 70000 lbs, but its certainly not a standard ford 9", and I think it has something to the tune of 12 or more bolts.
Could anyone give me a real world breakdown of what steps are involved, and can I easily do this with the rear axle on the truck? And is there anything special regarding the dually rear end?

[Jacksdad]

Yes, it should be a Dana 70 if it's original. Some owners here have upgraded to a Sterling 10.5" rear end from a later Ford dually. It's stronger than a Dana 70, and they can be had with limited slip diffs. I have the same gearing as you, so it's a swap I'm considering down the road.

[bwlyon]

If you have a T18 or a NP435 4 speed, have you considered a 2 speed Ranger Torque Splitter that goes between the bell housing and the transmission. It adds 27% overdrive, which would reduce RPM to a reasonable level at highway speed, and would add 4 new gear ratios. There is a write up on this website. Of course the drive shaft will have to be shortened 7.5" and new holes for the shift levers and such, but is a viable option. Plus a "two stick" transmissions are cool! Advanced adapters still make these units.

[Busboy]

Search E-bay for a factory Dana 70 service manual. I rebuilt one at work in an E-450. Being a machinist, this is right up your alley. I highly recommend buying or borrowing a diff spreader just as the Ford and Dana manuals say. And if you have a dumbass boss that thinks he can build one out of angle iron and all thread instead of buying one, tell him to go pound sand. Seriously it's just precision mechanical work, pinion preload is important and following the pinion nut torque spec is important. I would definitely record the position of all shims when you take the thing apart.

[Ranchero50]

OK, been down this road and doing gear swaps in our Dana 70's doesn't work. Basically the carrier housing comes in two configurations, one that uses 4.10 and higher gear and one that uses 3.73 (I think) and lower. The 4.10 ring gear is thick and the 3.5 is thinner, doesn't fit... It took a while to figure it out and I was tipped off to it from a gear vendor up at Carlisle. The Dana 70 changed later so it could run taller gears.

I went with a Sterling 10.25" but they use a larger drive shaft yoke for the larger U joint, 1410 I think vs. the smaller 1350. There were some adapter U joints but I couldn't come up with a combo from NAPA that would work. My Dodge sourced 47rh drive shaft used the larger U joint so I welded up a complete shaft. It was kind of a PITA.

There may be some more details in my build thread.