The 5.2L I4 IIRC is about a 200/400 motor. So ANY motor with those specs is going to be ok in town, on the freeway depending upon the what and where a bit sluggish. Mid to upper teens for fuel mileage is about right. These are city drivers, not geared for OTR work. Local work only. It would take some reworking of the drivetrain gears to make them true freeway drivers.
The I6 should work ok in the cab over or conventional cab class 6 and above trucks. Again, depending upon the HP/torque specs and gearing. Unlike the 4 banger where there is ONE power option, the I6 has aroudn a dozen HP/torque spec options depending upon the end users use of the truck OTR vs city use etc.
I've driven a number of the Class 4 cab overs since they first came out with a 3.9L 125/250 motor. The new ones are better on the power hopefully obviously, but if you want to keep up with one of the pickup diesels, never going to happen! That is not the design, nor has it ever been the design, nor would I imagine it to ever be the design of that truck.
Marty