Anything is possible with deep pockets..

Gotta be the st170 6 speed but there is no conversion kit available, and it wont be an easy mod.. Probably easier to change to an st170 engine and box tbh..

If you want lower motorway cruising revs, you can do a converion to an mtx-75 gearbox (5-speed) but change the ratios and final drive.. Therefore allowing for lower revs at higher speeds..

Si (might be SiST on here) is your man for that.. He offers full conversion kits at very reasonable prices considering the amount of bits needed.. He can also talk you through the fitting.. The mtx box is a hell of a lot stronger than the standard ib5 so is the obvious conversion choice for big power application..