My Two cents:
1. Ask John Taylor to give you an estimate on a reline to match the Existing designation, 38-55, using a TJ Liner or other liner as recommended by John Taylor of Taylor Machine.
He has done work for me and I am well Pleased wiht both the timelyness and reasonable costs o fhis work.
2. Buy a Barrel blank and have it machined to fit your action, chambered in the caliber of your choice. I am slightly Prejudiced toward Lothar Walther barrel blanks as every one i have purchased has been within +/-0.001" of stated Bore/Groove diameters when slugged.
From your description of your Stevens '44-1/2' it sounds like you traded for an excellent single shot action of moderate strength. Moderate because it is NOT suitable for the Various Rifle Magnum cartridges and is most comfortable with Middlel of the road pistol calibers or target Rifle calibers of moderate velocity and recoil.
I do not believe "O' stamping indicates a thinner, smaller, receiver as several of my Stevens actions have that Stamping and measure equal to others without such Stamping.
Good luck wiht your project.
Chev. William