There is no published towing capacity for these old trucks. And I've not found any standard 'formula' that would give you a good number.
I believe towing capacity is the combination of several factors, any one of which might limit the capacity.
The first and most importance is brakes. If you can't stop the load you should not start pulling it. When you see towing capacity on a modern vehicle its usually got as much to do with the brakes as anything else.
You say nothing about rear axle or brakes in your question. Far more than the V-8 and automatic, your decisions about brakes will determine true towing capacity.
Second, in my humble opinion, is stability and handling with a load. This means attaching a hitch and trailer does not compromise your ability to control the vehicle. I think this means the suspension and steering all works properly and the hitch is located in a way that attaches the load without adversly impacting handling.
Third is can you move the load. Since trucks like yours have been used to move some pretty heavy loads, the answer is likely to be yes.
But if you can't stop it, and can't control it, you don't have the capacity to tow it.
Those are my thoughts, I'm sure someone will come along and say I missed something. They may be right--I'm on my second beer...
I think the answer regarding increasing towing capacity is an extension of the above. Increase braking performance, improve handling, and improve power.
I'm bulding a '55 1st GMC 1-ton to pull a trailer so I've spent some time looking for the same kind of answers you are. I didn't find much. My decision was to build something that meets or exceeds in specifications those of something that will tow the load I want.
My target is to tow 7,000 to 9,999 across Wyoming at the speed limit (75 mph) and feel like I can pass if I want to. The last time I towed across Wyoming I was driving a 454 powered 1-ton 4x4 that weighs upwards of 6,000 with a trailer and load adding another 6,000. Figure a combined load of nearly 12,000. My plan is to out perform that truck.
I expect my finished 1-ton to weigh less than 5,000 and will power it with a 500 Cadillac. Transmission will be a TH 400 (same as the 4x4 Chevy) and rear axle will be a GM 14-bolt full float similar to the Dana 70HD in the other truck.
Brakes on the 14-bolt are the same size as those on the Dana and that's what I think matters most. Otherwise the Dana might be stouter than the 14-bolt. However the 14-bolt has 3.42 gears which I think are near ideal for my purposes. It won't have the same grunt down low as the 4.11 gears in the Dana, but with lower weight and more torque I expect to be okay.
Up front I'm installing the IFS from a '72 3/4-ton. Starting in '71 these suspensions came with disk brakes. If yours is earlier and has drum brakes I'd look for another one.
The part numbers on the brakes on the'72 C20 match those on a '72 C30 (single wheel) in the catalog where I checked things, and since I believe the brakes are the most important part, I feel those brakes will be adequate.
My IFS was under a big block truck so I'm not worried about using it under the Cadillac motor.
I hope I haven't bored you with my story. I told it here to illustrate how I think you can increase towing capacity on one of these old trucks.
