the Getrag 221 is only ratred for 240ft/lbs of torque - over the V6 but well under the V8's output. Not to say that you could not strentghen the internals as a one-off project, but for a OE manufacturer it was impactical. As stated above, the dealers to Ford there was no demand for a manual V8, hence the lack of development.
There are many threads on this... but in short:
- custom bell housing
- custom PCM tune (if even possible) - the big issue
- custom drive shaft
- custom gearbox mounting
- swap wiring harnesses - mating V8 harness with manual transmission
- exhaust swap
- all the bits with the engine swap....