Vacuum operated would work too, if you can find a suitable switch.
The concept is that the A/C only sucks power from the engine while the compressor is running, and the compressor only runs while it's clutch is supplied with 12v. So if you insert a switch into the wire to the clutch, such that it opens when you want full power to the wheels, it will shut the compressor off and stop it from robbing power. Of course it will also stop cooling the interior of the car, but it will take some time to warm up and hopefully by then you'll be off the throttle.
I'm not familiar with the throttle linkage on your Spit, so I can't give any specifics. But the idea is to build a bracket that holds the switch, so the switch operates when the throttle is nearly open. Letting the lever for the pedal hit the lever for the switch might be the easiest way, but you'll need to use a little imagination. Hanging it on the side of the carb could be easier to get at (but also more cluttered looking with wires & whatnot).
Lots of switches would do, but be sure the one you choose is rated to carry the current of the A/C clutch, and there is a suppressor diode on the clutch to reduce kickback. The optimum solution might be to use a relay (if you don't have one already), but a bare switch will do. If you switch the clutch directly, the wires to/from the switch also have to carry the clutch current.