I have worked on a few original 68s with the Powerglide. The kickdown is a rod set-up, and there are 2 different lengths, a longer one for the 2 bbl and a shorted one for the 4bbl. In June 2012, we finished the 2 year restoration of a 68 El Camino. We had switched from the stock 307 (originally a 2bbl and converted to a 4 bbl about 10 years ago) to a 68 327/factory 4bbl motor. Initially, we used the kickdown rod from the 2bbl, primarily because it was handy. There had an added stop (a nut and bolt) part of the way down the slot that connects to the carb. When we did the initial 4bbl conversion on the 307, we had added the stop. Ten years later when we did the 327 change over,we could not remember why we had added the stop, so we removed it because it was not shown in the Assembly or MX manuals for the 68. This would turn out to be a major goof-up that led to 6 hours of trial-and-error adjustments that went nowhere! We had a dickens of a time trying to get the trans to kick down with that rod. We took the car out on the highway, got up to a cruising speed and nailed the gas pedal in an effort to get it to kick down. When it didn't kick down, we would pull over and adjust the rod, and then do it again. We did it continuously until we used up the entire thread length without getting it to kick down. When we went back to the shop adn thought this through. The lightbulb went on; we had the stop in there for a reason. SInce we had the other kickdown from the 327/4bbl setup somewhere in the shop, we decided to find it to see if there was a difference, and there was. The original one for the 327/4bbl motor was shorter. In hindsight, we had figured that out years before. That is why we had put a stop nut and bolt in the first kickdown rod back when we converted the 2bbl to a factory 4bbl set up. Both of us being "chronologically enhanced" (Old men!), we had forgotten this issue.
Here is info from the 1968 Assembly Manual about the 2 different rods for the SB motors. Check out the Assembly manual in section M-35, page A-19. PN 380310 for the 307 *(a factory 2 bbl) and PN 3904060 for the L-30 and L73 327s (4bbl motors).