the tranny cross member will need to be moved, doesnt line up with your old mount location.
Its heavy too.
why bother, even if its free, c4's are cheap and easy to find.
ive got one in the garage now, i think the tranny shift arms are bassackword (thats backword) from from c4.
tranny length is different ,so you need to change drive line , more than likely a diff front yoke too.
less parts available/info for it.
As has been stated above, why would you want an FMX? They are big old slush boxes with a relatively poor reputation. I would either use a C4 or an AOD. The AOD would't be really any more difficult. Plus there are conversion parts and info available to do the swap.