Thanks everyone for all the comments. Steve, I too use only D-rings, as I don't like the protrusion of eyelets from the back of the picture, and it's too much work to try screwing them into the inside of the frame (and I risk messing up the stretched canvas).
Here is my piece in place:
Recall that it is 72W x 36H. I also had two vertical cross bars, giving me 4 vertical supports every 24".
I put a single-screw D-ring on each support, about 11" down from the top. I threaded a single wire through all 4 D-rings, with enough slack so that the wire, when mounted on hooks would look something like this: |^|-|^| where the vertical lines are the vertical bars of the frame. Because only a single wire is used, I could adjust the level of the picture without having to have the two mounting hooks perfectly level.
I only wish I had gone up a notch in strength of the mounting hooks I used on the wall. Once hung, the picture is okay, but during the hanging/leveling process, those hooks undergo extra stress.
(Oh, and when you are buying D-rings, I found them at Target for about 1/5 the price of Home Depot.)
Thanks again for the comments and input!