I believe the only gears numerically lower that will fit you current carrier are 3.36 gears.
Carriers went 2.56-2.78,/ 2.93,3.08 and 3.23 then 3.36,3.55.3.90,4.33. I got this info from the 67 service manual.
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67 GTO HT
1968 400 Engine YE
TH-400
#13 Heads -J069
Q'Jet .
8.2 Eaton Posi/3:55
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