Bud - Any conversion, Ford or Datsun, requires a kit. Both the Moss kit ($2700) and the Rivergate kit ($1800) are complete. With the Moss T9 conversion, you get one of their trannies, even if you already have one. With the Rivergate kit you can get the kit with or without an included transmission.
I am getting ready to install the Rivergate kit using the '81 - '83 Datsun 280zx transmission. I chose this kit because it's almost $1000 less than the Moss kit and I chose this transmission because of the .75 5th gear ratio Rick Ingram mentioned. (The T9 has a .84 ratio, roughly the same as the MG's O/D tranny.)
In my case I'm providing my own Datsun transmission ($113 from a salvage yard). Doing this the kit and tranny will come to about $1150. Pretty easy decision actually.
Hope this helps. Cheers!
R.