how hard is it?

How hard is it to rebuild an FMX tranny? Anyone know what special tools are needed and where I could get a rebuild manual?

There are spacers for setting the bands - but you can use a drill bit for that. There are piston installation tools that make putting the apply pistons into the drums a cakewalk but you can use a wire piston seal installer (if you are patient and careful). You will need a micrometer and a dial indicator with a universal mount of some kind. If you are doing a thorough rbuild you will need a set of bushing and seal drivers. You will need a spring compressor for the clutch drum but it can be made from a couple of short pieces of flat bar and some threaded rod. Most automatic transmission parts warehouses have ATSG manuals (Automatic Trasmission Service Group). You might be able to find a video on "Bad Shoe Productions" website. Motors manuals makes transmission service and labor guides.
You are going to need a lot of space and it needs to be clean. You will need some way to clean parts and air to blow them off. You will need a jar of Vaseline to use as assembly lube and a quart or two of fluid. the fluid is used to soak the clutches and bands in before you install them and to lube the washers, bearings and shafts as you assemble the transmission. LOTS of shop towels!
I also use Nitrile gloves to keep the black crud from permanently coloring my nails and hands. - it coats everything inside the transmission. A pressure washer is nice to clean the case but you can scrub it with soap and hot water and then wash it down with solvent.

Sounds like I would ship it to Paul! Toploaders are much easier.

Continuing on from Paul's comments:
The ASTG manual is available *everywhere*.
I use the 1970 Mustang manual -- very complete, I like it better than ASTG which is essentially a reduced version of the Ford manual.

The clutch spring compressor is made by OTG and can be found on the internet.

The seal protectors are available from several sources like bulkpart.com

The rebuild kits are still available from Napa, Bulkpart, and others.

The FMX requires more patience than a C6 in my opinion. The pressure feed tubes to the regulator from the valve body are stubborn. Be gentle.

I did my first FMX in 86 without any special tools -- I made everything. It made it trickier than with the correct tools which I have now.

The FMX ran great for years until I yanked it out.

There is a Ford transmission forum with some FMX gurus out there.