I would suggest buying 5/8" sheet rock and put that in place, make a 1-1/2' cutline into the paper of the existing sheet rock on either side of the opening and remove the paper. This will allow for your "paper tape" to sit somewhat flush to the existing wall surface.
Keep in mind, it is much easier to builed up the mud to the existing surface then it is to sand down the build up of mud in the end. By using sheetrock which sits 1-/16" to 1/8" less than the existing wall surface,this allows you to gradually add mud to match the existing surface. With the tape already set in, this will eliminate an obvious "built-up" in the end.
Keep in mind that, when it's all said and done, your probably looking at an area of dry mud that is about 16-18" wide after feathering.