I think that the Runco 980 is based on the NEC XG135, but you would have to experiment around to see if you have the horizontal sizing range to make this work.
It's easy to test out. Just see if you can adjust down the picture width on your current screen to 1.33:1 width (which it should do easily), and then adjust the picture width past the edges of your current screen to the full 2.35:1 proportion, keeping the current full screen height. You can use a non-anamorphic 2.35:1 DVD to get the proportions right for testing.
As far as the burn in goes, it doesn't have to do with the screen, but the projector's CRTs. The CRTs are 4:3, with the smaller, static height image you increase the chance of burn-in and reduce brightness and resolution. But as you mention, having a double-stack, your individual projectors will be less pushed, so burn-in will be less of and issue, especially with a reasonably sized screen like yours.
So the biggest obsticle will be the sizing control range, and I know it is possible with some projectors, so you will just have to test yours out to be sure.