Can a 78 Volare cruise control setup work in a 74 duster?

Just working from memory, I think your guesses about the main power wire and the double plug going to the brake light switch are correct.

I could see a relay working, just remember that you are in effect setting up a controlled “stuck throttle” condition. If care isn’t taken and power on the control wire isn’t cut when the brakes are applied, the module will assume you are going up a hill or something and apply more throttle when you are trying to slow down. So whatever method is used, it should be wired into the brake switch and clutch pedal (if there is one). I would strongly suggest a toggle is never used.

I actually disassembled a couple of stalks to build one good one. Found one with good wiring, but the angle on the stalk was wrong, but all the stalks that would work had wiring that were chewed up. I made it work, but there are some contacts that are pressed into the main body of the stalk and while I got them back in and it is functional, they didn’t go in as nice as they were before I took them out.

I’ve got parts from two other stalks left over, but no wiring that is great. And only one stalk with the right angle.