Automatic Transmissions make more torque than a Manual. By a wide margin. They have a torque converter PLUS the torque of the gear ratios.
But Automatics are MUCH more expensive to build and maintain, make more heat, require an extra computer in today's world and are heavier.
In over-the-road trucks, cost is important. Very important. Automatics are thousands more to buy and maintain. And with the constant up and down-shifting, need to be rebuilt quite a bit more often.... About twice as often. And that takes the rig off the road. A cardinal sin....
An automatic transmission is the better choice in a passenger/light-duty vehicle application. It just is. Period. Unless you enjoy shifting gears all the time.
On a Semi, the rig owners don't really care if the driver is up and down-shifting 20, 30, 40, 50 times an hour.
And the Ford 4R100 is one of the BEST transmissions of that era. Period.
It was rated to handle 1000 lbs of torque but when people start modifying their diesels to make crazy amounts of power, they'll fail. Like any of them.
And if that 6-speed manual is a ZF? Good luck trying to rebuild it if you ever have to. Just swap it out and be done with it. Been there, done that, burned the T-Shirt.
But I can see the attraction of a Manual. Nothing wrong with them as long as you know what you're getting into.