I thought I would share my experience with others in case they run into a similar problem trying to time a 235. Every time I attempted to turn the distributor so the steel ball was centered on the flywheel mark and went to tighten down the distributor, the steel ball would once again disappear. This afternoon it finally hit me what was going on. My vacuum line that is connected to the vacuum advance is metal tubing. There was sufficient tension on the tubing that it would turn the distributor slightly by putting pressure against it and subsequently throw the steel ball off as I was tightening it. I could not find anything in the shop manual indicating that the vacuum advance is supposed to be disconnected while setting timing, but I disconnected it anyway. That allowed me to center the timing mark and tighten down the distributor without it moving. I could then reconnect the vacuum line and the timing mark stayed centered. Quite an ordeal, but it turned out to be a simple solution. I may ultimately replace the metal tubing vacuum line with vacuum hose.

1953 Chevy 3600 3/4-Ton