Terry Haines is the best guy for MTX75's though. They have changed the internals a lot, and some parts are meant for 4cyl and others are V6. Some can be used together and some cannot. Since he was an engineer with ford and knows the MTX75 like the back of his hand, I had him build mine.
With a torsen LSD, upgraded shift tower, upgraded shift forks, upgraded gears and syncro's it came to about $1500 to rebuild my trans.
check out
www.car-parts.com for a MTX75. You can find low mile units for $400-500 and it'd be under $100 to ship. Look for non-SVT units as those are usually likely not to be beaten like an SVT or maybe a cougar. 2000+ model year units are the cream of the crop, but remember to get a V6 trans. The difference in the newer Cougar units is the shift tower that was used with them, but that could be changed out to the correct upgraded shift tower if need be.