I'm leaning with 71' Duster 340.
More nimble, and more consistent off the line.
In 'stock vs. stock' with a 4-Speed and 3.91's - The Duster 340 can pull out early
over the wheel-hopping Mustang, even with F-60 x 15" Goodyear Polyglas GT tires.
Those Big-Port ~ Big-Valve Cylinder Heads won't be effective until mid-Track.
By then, the Boss 351 will be a good 3-Cars behind. Too much ground to make
up on the Big-End.