For quilts over 50" or so, I use a split sleeve so the rod can be supported in the middle as well as on the ends. I simply make two shorter hanging sleeves (instead of one long one) and leave a gap of a couple inches between them. I usually use a narrower dowel and support it with a cuphook or nail in the middle; you will need something a little heavier to support such a thick dowel.
I wonder if it mightn't be better to use a piece of wood - like a 1" x 3" trim piece instead of a round pipe. The quilt would hang flatter (no round lump at the top) and you could support it with a nail in the middle.