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It's really about the torque more than the hp. A 7.5 with good traction against 400 ft/lbs loses, regardless of the rpm the engine's spinning.

Different gear ratios also make a difference. The ring and pinion ratio multiplies the torque coming into the rear end. 300 ft/lbs through 3.73 gears creates 1119 ft/lbs at the axles. But 400 ft/lbs through a 2.29 gear only creates 916 ft/lbs at the axles.

This is part of the reason why the 7.5 in my '79 Monte survived for 8 years behind a Buick 455. The 455 was only producing about 370 ft/lbs and the rear had 2.29 gears. When I replaced the axle it was due to wear at the axle bearing surface.