I ran some really quick and dirty calculations the other day, and it looks something like this:

stock 2.5L - 63mm
svt 2.5L - 65mm
3L w/ svt upper - 70mm

Ideally for max power you should run a TB that is just large enough to feed both intake runners during high-rpm operation. Any larger and other crap may come into factor (i.e. driveability concerns at part throttle, etc.).

\'94 Cobra #4963/5009, black on black, not quite stock
Formerly owned a black '00 SVT, #1972
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RIP - Ray "Old Fart Emeritus" McNairy