A 6 liter has plenty of power/tq.
In WW2 those trucks you see hauling and towing large loads are just 6 cyl gas engined- probably made well under 120hp( they were no doubt big 6's-plenty of low RPM torque-but not as much as a 1998 5.7).
A 5.7 makes 250 hp-plenty enough motor to tow.
The motor/gearing determine how fast you can accelerate with a load.
If you don't mind very slow acceleration the motor isn't the problem.
Now all the rest of the running gear and BRAKES and weight/wheelbase of your vehicle have lots to do with towing safety, and reliability.
A 3/4 ton drivetrain in a big long heavy wide vehicle like a Suburban should be up to handling 9000 lbs or so.
Now some older Suburbans- my 1998 1/2 ton- have kinda whimpy brakes- I don't think I would care to tow more than 3500 lbs or so with these sissy brakes.
I would get some sort of aftermarket brake kit-rotors calipers etc- if I towed much at all.
The motor is rarely a limiting factor in towing- all the other stuff is-size/brakes/trans/gearing/weight/wheelbase/track etc.Light Little short wheelbase vehicles don't tow well.
PS- Diesels are great tow vehicles, but they probably aren't overall cheaper to own. The FI systems are expensive to repair,and seem to regularly break.
A SBC is cheaper to buy cheaper to own cheaper to repair- a BBC- probably cheaper also- but very thirsty even when not towing.
Most folks don't run their vehicles past 200,000 miles, so how long a diesel can last isn't too important.Plenty of SBC last 175,000 plus miles.
Diesels make better tow vehicles, but they aren't cheaper than a SBC or a BBC- even considering the HUGE thirst of a 7.4 or 8.1.
The whole point of the bigger small blocks in 3/4 ton is to have an inexpensive to buy and own tow vehicle. The Duramax option adds $6000 or more to price with the expensive maintenance to boot
Resale is pretty good on diesels, but they are so expensive to buy and own(new)