I think MB's design engineers were out to lunch when they designed those rails. They play no active part in the running of the motor. The rails are there to keep things in line when the chain goes slack.
The Aluminum rails were the designers choice in the original v8s 3.5,4.5, and 6.9. I would gladly use the original MB Aluminium rails if they were still available. I haven't seen the aftermarket replacements so I can't say. I can make a good guess though; that the aluminum rails wouldn't fracture like a beer bottle no matter how old they got. When the rails get about half the color of a beer bottle they are about as fragile. The only way you could grind up a aluminum one would be to run to motor in constant chain slack.
I doubt anyone on these forums would do that.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician