I would do it right and go to the 70mm....
But if your lookin for more productive mods:
Look at 3.73 Gears, pulleys, and even a bigger MAF may let your horse breathe better than just a throttle body alone...
(It won't matter how big the TB is if its sucking through a straw...
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98 GT
Bassani X-pipe, Dyno-Max Cat backs, 3.73 gears, Steeda shifter, FMS pullies, 87mm MAF, Roush cold air kit, Eibach springs, and Zex 75 shot