i dont think you would be able to. because if you cut the wires for the auto then it will receive a faulty reading, and compensate for its reading by telling the rest of the computer what its doing.
sorta like when you pull a vacuum line off a skyline (it revs up something cronic, and if you put your finger over the end of it, then it stalls)

so my advice would be buy a manual computer. theyre called manual or auto computers for a reason im guessing...

you will also therefore need a manual loom, a clutch pedal and master cylinder. also a gearbox!!

When changing from auto to manual do you need to change computers or can you just rewire the auto comp? -bypass the auto in park to start sensor