There is a difference between out Mark half shafts and all the others. When I lived in FL I dealt with Raxles a lot.
First off, you stock shafts in the Marks are the strongest ones built, then comes the Tbird SC and then your standard V8 MN12 and last the V6 MN12s.
I have seen and done it myself on my own Mark and Tbird, the splines will twist until they snap. When I broke my last set on the Mark they twisted almost 3/4s around before breaking. I couldn't get the hubs off because of this and had to replace them. Since I had to replace them I went ahead and did the Cobra conversion.
The Mark and SC shafts have extra hardening treatment making them stronger besides being larger in diameter. But the real weak point is where they neck down for the splines.
Anyways, talking with Raxles I found that they used the Mark splines until they ran out and then would use the Tbird SC units. I offered to send them a set of good Mark shafts so that they could build me a new set but they wouldn't do it and so sold me a set of SC units. They then told me that the Mark shafts were getting too hard to find since the warehouses were being depleted and they expected the same with the SC shafts soon afterwards. When this happens they said they would stop building these shafts for us. Looks like this has now happened.
As for Ebay stuff, I bet you that those are just V8 and V6 Tbird shafts. If they are they won't last long under some serious power.
