I'm co-owner of a 2-man, 1-truck towing business along with working at Pepsi f/t and taxi p/t. Our current truck is a '91 Ford Super Duty(14,500gvwr), 7.3L non-turbo, ZF 5 spd, 4.56 gears(I think), and weighs about 11,000 pounds w/no driver. With a car on hook your looking at close to 15,000 combined weight, even more with a 3 ton+ pickup on hook. It turns about 2850 rpm at 70 mph. Factory ratings are 180hp and 345tq. Now, the engine that's in it runs fine and is OK for power but could really use more. We could always just put a Banks turbo kit on it but ya just can't beat a Cummins when it comes to reliable, dependable power!! Oh yeah, awesome cold start-ability too!! My ultimate dream if this engine were to fail or we got the time and money(money more than anything) to do it would be to do a CTD transplant. Of course, a CTD would have to be located first. An I/C one would be best but a non-I/C swap would be simpler. Here's a few upgrades that I've thought out so far as to what would make the Cummins a worthwhile upgrade.
3200 or possibly even 4000 spring for high speed(legal) highway/interstate cruising.
Tweaked pump.
Would a smaller turbo hot side be necessary for quick spool-up or would an 18.5 be sufficient with the 5spd and deep gears? Reasonable egt's MUST be able to be maintained when its hauling a car up a long grade with it floored. On second thought, with a properly setup CTD, WOT may not be necessary to maintain speed with a 7-8 ton load going up a grade.
4" exhaust
BHAF
Exhaust brake(optional)
Reasonable mileage must be maintained. Like low teens empty.
Any other suggestions that would make the Cummins an awesome towing beast for the 5 1/2 ton truck it would have to move around?

Right now I can only dream about this conversion, however, later on down the road when an increase in business calls for the need of a 2nd truck(rollback), I would really like to seriously entertain this idea.


This is a pic of when I had to tow a 2nd gen, 1-ton(duals removed), 2wd, 5 spd Cummins out of Menards because it wouldn't start. This was over a year ago long before I bought my CTD. He wanted it taken to the truck stop, well they really didn't have a clue. I called a mechanic at the Chrysler dealership whom I'm familiar with, told him what the deal was and he told me how to lift up on the bottom of the fuel shut-off solenoid so it would start. The guy was real happy and I showed him how to get it started until he could get it fixed.
And yes, sitting behind us is a 3rd gen.

 

PM bearkiller about his setup. His setup eats the weight you're talking about for breakfast

 

Stick with the 18.5, geared the way it is and with a 3200 RPM spring you will be able to keep 'er wound up and spooled. Depending on how fast you want to run, you might consider upgrading to the 60 # valve springs to prevent valve float. You'll need them for the exhaust brake anyway - and running at 15k you really ought to have one.

Also consider upgrading to 19.5" wheels.

 

X2, I never had a problem with spooling my 18.5 when I had my 1993 D250 5 speed, nor did I have a 3200 spring. As far as egts my ran hot beacause I was too stuburn to turn my full load screw back. But when I decided to bump my timing up, and it lower my egts to about a 1100 until I threw PODs in there.