This sounds like a feature request to have an option to make the 3D Box tileable. (Which I believe has been made at some point. A checkbox for "Repeat Texture" would work, and if unchecked it would just stretch like normal.)
As for making this without the 3D box... well, the Sprite has a skew action that you could use. But unfortunately the Sprite doesn't tile either. Then again, sprites are a lot less GPU intensive than 3D Boxes, and there's also talk of Z Depth options for sprites in the upcoming build, so that might make things easier.
My advice would be to use the 3D Box... but just make different sized boxes that you could use. Like, a 1x1x1, a 2x1x1, and a 4x1x1. That way you could manually make a repeating texture for the longer boxes. And having boxes that are 4 units long instead of 1 would take fewer boxes to build the levels you need, and having the smaller sizes will help fill in the details.
Here's another tip: For some reason having a lot of 3D boxes in your level will affect FPS even if they're not onscreen. Turning the visibility off on any boxes that are a little bit outside of the viewable area will improve performance a lot. This is something I discovered with Arsonide when I was helping him with his game