Re: 300WSM Length and Twist ???
1. Some one of a kind bullets require special throating which works great for them and might not work well for "standard" bullets.
2. Faster twists will cause higher pressures sooner so you will give up velocity as compared to the same bullet in a slower twist. I can run 210s out of a 11 or 12 twist faster than a 10 twist, proven it time and again. I have had all three in WSMs.
IMO you are smarter figuring your max weight bullet and twist that will work with that bullet and then building. that was stated by others also.
For example, very few are shooting the 240s in the WSM or Win Mag so why twist for it with a 9 twist? You really need a larger case to push the 240s right, and when you do that you can stay at the 10 twist. The 9 twist was originally for NRA 1000 yard shooters who were running 240s at 2650-2700 fps if that.
Particularily for a first time build, this can be limiting.
Anyway, just my opinion.