Need some old timer advice please,

My thought wouod be to change the pumpins as It would be the easiest in my opinion. You wouldn't have to change drive shafts or anything else. Check with a truck salvage yard and see what they have for 3.70 or 3.55 and see if they will take your old rear ends in trade. They should also be able to tell you what speed you would be running and where your rpms would be at with the different gear ratios. As far as rpms I believe you want to stay above 1700 with the big cam cummins.

 

I’d go with the Allison, 1500 rpm’s with a max gross of 44,000 will not hurt that engine. You’re not going to be working it that hard on those flat Kansas roads.

I have a friend with an old freightliner cabover that he turned into a toy hauler and travel trailer puller that cruises around that rpm with a big cam 3, I think. It’s a 3 or 4, can’t remember but he was really surprised at the fuel economy gain.

 

If you want more speed can't you swap the gears over to make it a double over? 4-5/8-9 reversed?

 

I got 75 @ 1600, 70.5 @1500.

 

It sounds like the cables are your biggest concern, but unless I am mistaken you can swap them out for rods, maybe not on that particular rig.
I am not a fan of the allisons for highway rigs, there is a reason that many old timers never drove one, juice trannys just didn't hold up to the day to day rigors of highway use. They did have some in the 70's, most that I personally saw were behind buzzin dozens, and they were never popular at all.
All that said, I am sure the new ones are much improved over the old ones, and should hold up alright for your use.
As far as 15 or 1600 rpm cruise speed, you should be fine with a big cam, just downshift when in a pull. I put a lot of miles on big cams that set at 1500 while cruising, with never a problem of any kind. These werrte big cam 3's not the 4 like you have though.

 

Lmao ...flat Kansas roads ..