I want to polish up my freshly removed mt75 gearbox. if i am to shine it up how long will it stay shiney - can i treat it with something to make the shine last, i really don't want to paint it silver. thanks

To be perfectly honest i wouldnt do it. Best spend time in other areas. Its largely hidden and covered by bonnet. Whilst the casting will be mechanically good you may find it isnt good enough to polish. A good clean and paint will make it look good (have you tried plastikote chrome spray?).

As an aside i sent my steel bits to a company that specialisies in de-rusting and zinc plating. I etch primed and sprayed silver smoothrite and they came up well (and that included brake drums). I rubbed down the zinc off surfaces that didnt need to be coated afterwards. Only cost £40 ish cash. My place was Almnit Metals in Newton Aycliffe, Co. Durham.

Concur with the above. Unless you are replicating The Fast And The Furious, don't bother, it won't make the car any faster. And you'll only have to re-polish it in future unless you laquer it.
If you're determined, there are specialist ali polishing kits which you can mount in a pistol drill avail from internet suppliers.