I don't like cutting on the diagonal. That long diagonal seam can stretch and cause problems, and if you make a mistake while cutting it then it's hard to recover.
Whether you have enough to make the backing depends on what size (how much larger than the top) you need it to be. If you're quilting on your DSM you probably only want it to be 47x47, but if it's going on a longarm you may need it to be as large as 53x53.
Assuming you can use the 47x47 size, you can just cut the first piece to be 47xWOF, and then cut the remaining piece in half lengthwise, sew the pieces end to end, trim to 47" length, and sew that long piece to the original 47xWOF piece. You will then have a piece that's 47x about 60, which you can trim to the 47x47 that you want. If you need the backing to be bigger, say 50x50, the the side piece will have to have more seams, but it can still be done.