I too think the IH engine and chassis is a better fit for your desired outcome.
While a Dodge truck chassis could work, that's COE has a lot of wind to push, then go 70-80, add a trailer, and you may be disappointed with the lack of power and or poor response.
The 5.9 Cummins engines can be turned up a fair amount reliably and cheaply because of all the Dodge diesel aftermarket goodies available. Still probably not the best option for your working truck.
The other part that may not be so good about using a pickup chassis is the front axle and suspension is just up to the task of holding the engine (some older ones not even that) then you add a big COE body and an extra 2 doors right over the front axle. Not a recipe for success.
The 5.9 Cummins was, and may still be, offered in medium duty trucks, like a Ford f750, and some small Freightliner and Sterling trucks. This setup would offer a sturdy frame, axles, and great hydraulic disc brakes, perhaps even air brakes. Still will have a slight;y underpowered engine, it will probably do the job, but nothing to brag about.
Part of your conversion needs to be serious brakes to stop that big truck and trailer at speed. Stepping up to a medium duty truck chassis, and or axles will solve that.
I have done a few axle conversions and a diesel engine swaps check out my pictures below.
Grigg
