I would say 350 in 40 on private may be possible but you are going to have your work cut out for you. Hunting public land in 40 limits you to a lot less land and bulls so you likely won't be able to be very selective.
If you do an incredible amount of scouting you may have a better chance at 350+ in 2 or 10 but it won't be easy! There are so many factors that go into a hunt. You better be super good at field judging and be willing to pass up smaller bulls and go home empty if 350+ is your goal. A lot of hunters somewhat panic after waiting so many years for tags. There are so many factors that go into a hunt. A guided hunt on private land with an outfitter could be a great option for those that don't have a lot of time to devote to scouting and may not be efficient at field judging bulls. I would make sure you have lots and lots of time to scout and hunt the entire season if you decide to go unguided.