1968 GTX production numbers break down as follows per Galen Govier's white book..........

2door hardtop 17,914 total produced
17,083 shipped

410 USA built with 426 Hemi
359 with 4 speed
325 with 727 Auto
11 Canadian built with 426 Hemi
with no break down on transmissions

16,673 USA built with 440
6,902 with 727 auto
undetermined number of 4 speeds

1,026 1968 GTX convertibles built
917 shipped
36 USA built with 426 Hemi
24 with 727 auto
12 with 4 speed
4 Canandian built with no trans breakdown

881 USA Built with 440
375 with 4 speed
undetermined number with auto

I am not really sure why the trans numbers are undetermined, given you have a total in some cases of one type of transmission... these are just the numbers and breakdowns out of Galens book... The math seems easy, but the total numbers may only be a ballpark and not 100% accurate... Hope this info helps....