Glyn
Only heard of people replacing with round slide Dell Orto, PHB PHM etc.
Not sure there is a lot of room under the tank and frame for anything to tall.
All the square slide VHB on 650s & 2502c have very strong springs, and you need a strong wrist action. The triple carb on the 900Sei is lighter, and maybe is due to the lever system at the top.
Barry