I had until recently a ford 350 with a Detroit Diesel 6V53T (actually with twin turbos).
It had an RTO-610 behind it, and I really liked it. It was great for towing, rpm drop between shifts was about 400 rpms, so I could keep it over 1800 where it had torque and power.
With the 10 speed there was always a gear available that would let me go the speed I wanted with the power available.
For example, I hauled (flat towed) a 7,000 lb plus truck from WA to NY, in addition to about 4,000 lb in the bed of my truck, and 10,000 lb empty weight of my truck, about 21,000 lb total, and I did have the brakes to stop it all.
On I-90 in Idaho going through the mountains near Coeur d'Alene I was in 3rd gear running about 20-25 mph at 2,600 rpm. When I could I would up shift and go a bit faster, but if it got to steep I would down shift again. If I had only a 5 speed the shift up would have been around 800-1000 rpm drop, and taken me out of my power band, the truck would stumble and I would have to try again in an even lower gear than before.
Before you say I had no power, think about this. Through those mountains I was passed (slowly) by a few semi trucks, but I also passed some semi trucks, slowly I'll admit. That Detroit did sound good screaming, pulling, and climbing those mountians... On the down hills I was just slower than most of the trucks, at about 25-30 mph
Until you drive a truck with lots of gears that are closely, and evenly spaced, and easy and quick to shift between them you can't fully appreciate it, and perhaps you won't understand it until then.
So, if you want to use a 4BT to tow with, you will have the best luck with a 10 speed, because you can keep the engine at a speed where it makes power no matter the road speed. You won't have to hold it against the governor just before you up shift to have enough power to keep moving once you do.