We quite often see 250/350 owners who bit into towing 14k-15k lbs with the small block 6.2 engine. Many swapped for the same 250/350 with the diesel.
Just to add to MFL on point comment on tire/wheel diameters.
Also I've found with gas small blocks, and big blocks, tire and wheel weight can drag down their towing performance.
Example of a tires weight;..... a BFG Mud Terrain KM3 LT295/70-18" that weighs 74 lbs. Now add the wheel and the little V8 has over 400+ lbs of tires and wheels to keep the load moving.
Even the same brand BFG same tire type in a 17" weighs 67 lbs.
Yeah.... I know we have lighter weight tires out here but this is just a example of parasitic drag on the small engine.