What are you talking about? MD
-80s, then DC-9s, then 757-200s and -300s? Which is it man?
-80 certainly doesn't even have close to the cargo capacity of a 734, and probably not much more than a 733.
-80 has a constrained cabin cross-section that makes it a poor choice for a freighter, and its underpowered compared to the two Boeing airframes. I'm not even sure there is a current STC for an MD
Obviously, DC-9s are out of the question. They uplift so little payload, and FedEx is retiring relatively new 727s so I doubt they'll get the much older than that DC-9.
The 757-200 can only carry a few more containers than the 734, and at a much higher ton mile cost, and a higher capex outlay.
There are no 757-300s available for purchase anywhere in the world.