Hi, Steve. I think that you will be much happier with 2x4 framing and 2x3 would be the next choice. Don't forget that you have to put some kind of framing around the inside of the opening, so leave an extra 1-1/2" in the rough opening for 2 pieces of 1x?. Don't forget to leave an extra 3/4" in the top framing for a piece up there too. I calculate the rough opening at 49-5/8 since 1/8" should be plenty for the swing.
When you hang the doors, you will have to mortise for the hinges. Your old doors are probably already mortised so you will probably only have to only do the outside of the door jamb if you can get the same sized or larger hinges than the ones the doors were mortised for. These doors will probably be pretty heavy so plan on running longer screws thru the door jamb and into the framing. The screws that come with todays hinges aren't long enough.
That's about all I can think of without seeing it. Good luck. Henry in MI