13.7 @ 102 is just about what a well driven 96-98 well driven Cobra can do bone stock. You can do a lot better, a lot easier, and a lot cheaper, with the 4.6 Cobra engine. The fastest to date that a know of are, 9.9 NA and 7.something with a blower.
Ford actually has a concept Mustang with the Lightning motor and a T56 tranny. It really didn't work very well. Partly because the car ended up horribly unbalanced, and partly because the gear ratios in the transmission were off.
BTW, the 4.6 to 5.8 (or 5.0) swap is actually much, much, easier.
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