I recommend a 26" barrel for the 6.5x55, but a 24" barrel is OK. I have a 6.5x55 and a .264 Win Mag with 9 twist barrels. Both shoot 140 VLD's very accurately, but may not do so in all conditions. I have tested the 6.5x55 down to around 1000 feet ASL and 40*F.
An 8 twist is a safer bet. My 8 twist 6.5-284 performed better in the wind vs my 6.5x55 @ very similar muzzle velocity, which I attribute to Brian Litz's research showing that effective bc can be degraded by inadequate bullet stability.
I don't have any experience with a no-throat or short-throat chamber. My 6.5x55 chamber was cut with a standard reamer and performs very well. I don't see a reason to obsess over throat length for this cartridge. If you want something other than the standard 6.5x55 chamber, I believe the 6.5x55 AI would yield more benefit than tinkering with throat dimensions.