In my experience, with my '69 RS 327-210 Hp/Powerglide, it is a combination of not only getting the kickdown adjusted properly, but also ensuring that the throttle linkage is adjusted properly. The procedures in the assembly manuals are, obviously, for stock pieces and parts. Aftermarket carbs, manifoldes, linkages, etc can have an affect on transmission operation, although, the general procedure for setting upshifts and downshifts would still be as described in the assembly manual. Mine didn't work prpoerly either until I made sure that the carb linkage was adjusted properly (first) and then the kikcdown linkage - but rmemember, my car is totally stock and the assembly manual info was spot on. It is also worth mentioning that the transmission was totally rebuilt in 2004 as a part of the mechanical restoration process.
If I put my foot into it, the upshift from first to secont will occur between 55 and 60 mph. If I am cruising at 55 or less ant put the pedal to the metal, the transmission downshifts to first. Much above 55 mph, no downshift - you are above the speed where the transmission will downshift in order to prevent over-revving of the engine.