I'm very broad shouldered (wider than you), and can reassure you that if you book FC on Alaska or Delta, you will not encroach on the other passenger. I highly recommend you pick an aisle seat, though, the fuselage curves in and windows don't always align with your shoulders. The aisle also gives you a little more space to lean into if you felt the need, and less bulk to squeeze past someone if getting up.

You can buy two economy seats but you can't buy "Basic" tickets because you will need to have a prior seat assignment to get them together. Each airline has their own process on how to book them and I don't know if you'd have to possibly pay for 'preferred' seating on top of the main cabin prices. The second ticket earns no miles, which is part of why one FC seat is better.

And FYI I know it isn't applicable for a trip to Alaska, but Southwest has a very generous "Customer of Size" policy. You would book two economy seats following their instructions, and when you check-in they update your reservation to give you pre-boarding so you can select two seats together, and a "reserved seat" slip, and best of all they will refund the second seat!!