Which Tremec TKX Do I Get

Do you have a choice in od gears?

If the engine is in and running;
Just put a vacuum gauge on the intake and slowly rev her up, watching the Vacuum. When it plateaus, drop her back a bit watching for the vacuum to drop. As soon as it does, grab the rpm. That rpm is the lowest rpm at which your engine is becoming efficient. The valve timing is finally trapping the mixture in the chamber, instead of allowing some to return to the intake manifold. And that is my minimum cruising rpm for fuel-economy.
Now, at that rpm, the timing will want to be waaaaaaaaaay more than the mechanical advance mechanism can supply. At say 2600 rpm, the engine may be wanting 50 or more degrees. If you do not supply it, your fuel-economy will suffer.A lot. So that is the job of the Vcan. The slower rpm that you cruise at, the harder it will be to meet the engine's cruise-timing needs.

To figure out what she wants;
just rev it up to your chosen cruise rpm and begin advancing the timing, always maintaining the cruise rpm. When the rpm no longer rises with additional timing, at the chosen rpm, read the timing. That is what she wants with no load.
I build my curve to ~3* less, then fine-tune, with my dash-mounted,dial-back, timing retard box. You will not notice an MPG difference in those 3 degrees so don't spend 3 summers looking for the ideal number. like I did,lol.

But if the engine is not in and running, then I can offer only this;
Cruising at too low an rpm for what is going on in the intake,and with not enough timing, is worse for fuel economy than; a little too high an rpm with adequate timing.