My preference wold be to add some length to the side extrusions. The downside is that you need longer gear racks and linear bearings as well. But you don't give up any length.If you have any thought toward cabinetmaking, even if only for your shop, you will want the capability to cut dovetails. That will mean being able to cut off the front of your machine. I'm not sure how far the cutter in a spindle overhangs the frame on a CNCRP/Avid pro machine, but you will need a few inches clearance. If the pro machine doesn't provide that "out of the box," you will need to set the front crossmember back sufficient to give you room to cut dovetails. There are alternatives, but they usually involve making removeable sections of spoilboard. The loss of a few inches of table length made the most sense to me.
Or, you can just assemble it with the end crossmember set back, can't you?