Some on the grendal sight they speak of a reamer avaliable for the 6.5x39. It has been around for awhile. the disadvantages of the it are as noted above are case capacity .IMOA the the 6.5x39 should be the easiest to build and make work as the standard mag would work. It would be a reload only propisition and might require a form die as well.
there would be a performance loss as well. I personaly thing figuring out a mag and reamer for the grendal would be worth it unless you dont mind reloading.