What is the linkage called that connects to the carburetor and controls the rpms at shift (in my case, a three speed automatic)? And how do you reset it?

sounds like a kickdown cable, but i'm not sure what you mean by 'reset it'. it's based off of throttle position, not rpm directly. you can adjust it to kickdown earlier or later by moving the adjuster.

It's called a detent cable. Depending on the style of detent you have, you can push in a button thats on the cable itself and pull/push the cable in or out of the housing adjusting shift times. Or, if it's an aftermarket, you might be able to losen bolts or screws to adjust shift rpm.

You can also adjust shift points by adjusting the screw in the modulator valve. It's the one that the vacume hose goes to down on the side of the transmission. I can't remember which way is which, so you better do some research on that before you adjust anything.

Hope this helps,

Joe

The "thingamajig" At wide open throttle, the slider part should be adjusted still have about 1/16 gap/play where it fits onto the carb levers (the 'wingwong for a goose bridle')