I found an older post by Mark Kovalsky (2008) that may have answered my question, partially, on the capability of the stock truck’s braking ability using the engine/AT along with the truck/trailer brakes.
I believe Mark said by disengaging the OD button—turning the light off/OD off, the coast clutch (4R100) engages in gears 1, 2, & 3 and the transmission now drives the engine, allowing the engine to idle, and giving a breaking “effect” instead of the engine driving the transmission and No need for the t/c to be locked.
Turning on the OD OFF light (overdrive disabled) does engage the coast clutch in gears 1, 2, and 3. Whether or not the torque converter is locked the transmission will drive the engine in a coasting condition.
The problem is that engine braking
(not breaking - breaking is when something fails and has to be repaired) is minimal on a diesel engine. Yes, the compression is higher, so it takes more force to compress the air in the cylinder, but once the piston passes top dead center you get almost all of that force back as the air is allowed to expand as the piston moves down the bore. Big diesels have true engine brakes where the valves are opened at top dead center to vent the compressed air. This prevents the gain I explained above.
The engine brakes available for the 7.3L is more properly called an exhaust brake. It's just a gate that closes the exhaust when braking is required. The engine has to work much harder to pump the exhaust past this gate. That provides the retarding force.
In a gas engine, the throttle plate is closed when coasting. The engine has to work hard to suck air past the throttle plate. This naturally provides a retarding force that the diesel engine doesn't have..
You will have some retarding force without an exhaust brake. You'll have a lot more with one.
The 7.3L has a gate already in the exhaust. It's a warm up valve. Many people have added a switch to manually turn this gate on which works just like an exhaust brake. I did this years ago on a 7.3L and it made a huge difference.