Activating MDS

First off, MDS lifters are pretty problematic and are not as robust as the standard lifters.

The logic behind the lifters is much more complicated than a simple vacuum signal at a given TPS, TPS/MAP will cross that area at many different times during a typical street drive and you would be going in and out of "MDS-mode" on a typical drive. There needs to be a vehicle speed input included with some level of hysteresis to avoid this.

Would it work with a carb? Yea, you could get it to work, I would move over to an Arduino controller to give your system some more logic. I would have no problem actuating it on/off with my EFI system, however, the benefits are minimal [to me]. However...you're still on a carb, which is not all that fuel efficient anyway. If mpg is your goal, it's time to move to EFI.

Another couple mpg at highway speeds is insignificant to me and not worth the reliability concerns. You would be better off working on reducing drag on the car, it wouldn't be hard to better mpg improvement and make the car faster [win/win].

If you get a kick out of playing with different features, EFI also gives you some options. I have no-lift shift on my car, 2-step launch control and will have a real %slip traction control here soon...