I have a 250 - 350 Road manual (Spanish, downloaded from www.lamaneta.com) that agrees with your valve timing, and it also says the 350 has an Amal 930. I also have written down that those figures are for a cam marked SCR 2.5.
Whenever I have measured valve timing I have never managed to get exact published figures, only very close. I read somewhere (maybe the Velocette forum) that the important figure to get right is the inlet closing measurement. However I agree that your current settings seem pretty close, and you may only notice a difference on a dyno. Having said that, I have had to change the valve timing by one tooth in order to get a bike (250 Road) to run properly/tickover at low rpm. Unfortunately I can't remember what I started and finished with
![Sad :(](./images/smilies/icon_e_sad.gif)
For what it's worth, I would fit the 27mm carb as it matches the inlet port.
I'm not an expert, it's just my experience.