Torque.
I used to tow with a 350 (four barrel), TH400, and 4.10 gears. Occasionally would use a K3500 (454, sm465, 3.73 gears on 215/75R16). The 350 would pull if you kept it between 35-4500rpm. The 454 was gutless. A stock 6.5 is as well. However, if the 6.5 is healthy internally, $2k worth...