[Armrer]

The OE tracks are made from period British Butter. If the lower track fasteners get loose, or were never tightened properly, movement will work harden.. then crack them. I suspect that’s 100% of the problem. If they are bolted down properly, nice and tight, no cracking.
It historically has been a PITA to properly tighten the lower tracks on both 289 & 427 cars. Many original owners half assed it. I have seen that in person.