Which Tremec TKX Do I Get

You will not be happy with a 3.27. I had a 2.87 first gear TKO in my 73 Mach 1 with a 408W, 3:91 rear gear and 295/60/15 drag radials. The thing was an absolute beast everywhere, especially off the line, and the close ratio made it stay right in the prime RPM range.

3.09 and 3.27 type gearing, in my humble opinion are relics from the smog motor era (the king was the 80-81 Z28 with 3:27) where they made a car feel torquey off the line, with nothing after that.

My suggestion would be to order the 2:87 and enjoy the close ratio. You can always mess with rear end ratio if you feel the low end is not what you want. Lower rear end gearing is kinda the whole point of a 5 or 6 speed anyway...again just my opinion.