I believe they do. However, although a mechanical secondary carb, is more suited for use on a manual trans with higher numerically ratio rear gearing than auto, that still doesn't mean it's the ideal choice. Mechanical secondaries are more difficult to tune properly, and are better suited for track use where most of the driving is WOT. Other than bragging rights ("I have a double-pumper,") it won't provide any benefits for street use, and the vacuum secondary will actually provide better throttle response and can be tuned (because of the differences in circuits) for better fuel economy as well.

If there's nothing wrong with the carburetor you have, and you know how to tune it, don't waste your money. Unless you stroke the motor, or add significantly higher flowing heads and a camshaft, what you have will do nicely.