1-20 is fine. A 1-18 probably is too but the 1-20 has been proven over and over again its a good twist for this application.

A 300gr at 2800fps is pretty close to max without going to one of the slower powders. Most use H4198 and it works well. You might be able to get a little more speed with something like 3031. Pushing a 350gr that fast though will require some very cautious load work.

SML traces for bullets over 325gr are pretty much non existent.