Well, the Ford has a 24 valve V-8, compared to our 16. The Mustang makes it's peak torque and HP 500 RPM higher in teh rpm range. The Mustang has a wider space gear ratio than we do, and a smaller rear end gear, resulting in a numerically higher gear ratio.
Before anyone starts pointing to overhead cams and 3+ valves per cylinder as an advantage, it is slowly getting recognized by some in the auto media that GM made the right decision in staying with a single cam pushrod engine. Dual overhead cam, 4-valve per cylinder engines make more horse power per cubic inch, however, GM's engine design make more HP per installation volume, more HP per engine weight, and more HP per manufacturing dollar.
The Mustang has now incorporated variable cam timing to go along with its OHC design.
I totatlly agree with you on the weight and money aspects of pushrod vs. OHC. But, the question was posed, why is a 4.6 keeping up with a 5.7. I tried to point out all areas of difference. I think a OHC engine with variable timing is going to have a much flatter torque curve, and could have a higher average HP rating. As far as what I put with the gear ratios, I don't even know which is the advantage. But, I thought I would at least post the info so others could figure it out.