It can be made to fit with a bit of modification, BUT then the "original " parts will not work if you switch back,will be "too loose", besides the MK II upper will no longer allow the bolt to "stay open" on the last shot, as all the corresponding levers/parts are mounted on the MK II 'lower'.....this should be done as a "last resort" if you are trying to build one from "parts" ( put together ONE gun from an assortment of parts)
Folks had found over time, its easier to replace the complete UPPER, than trying to pull ,remove, or change the barrels alone, especially without the proper barrel wrench & receiver jig...........seen MANY "twisted" receivers over the years :roll: