i have a 77 t-bird with an FMX ,and just had it rebuilt last winter .
@ under $300, (it took a planetary set) .
a local trans shop owner & friend (my rebuilder). who cut me a break cause im friends w/his son (from my school days),incl.towing ,a new planetary ,(that took two weeks to get), a trans-go shift kit ,and rebuild ..but beware the fmx is known for being tough to find new parts or any performance stuff too , and ive been told by my rebuilder that they are known for the planetary problems , hope this was helpfull ...
iv'e had an Fmx transmission in my 70 mustang w/351 for 20 years that i have owned it, and not a lick of problems with it outside of a vacuum hose. it has been a very tough tranny. very dependable, i live in the west coast area, and they are hard to find.