The image below shows the airspace along the route. I have added a BLUE CIRCLE locating an aerobatic area for airplanes usually coming out of Redlands Airport. The center of the box is almost exactly over the DAM and the radius of the circle is 2 nautical miles. The vertical limits are 1500' AGL up to 7000' MSL. Here's what you should do. Just before launching Marshall, call 1-800-WX-BRIEF and ask about any NOTAMS for the aerobatic box NE of Redlands airport. If it's NOT HOT then go for it. If it IS HOT then you can either skirt the airspace, OR carry an aviation radio and set the frequency to 123.35 and you will be able to talk DIRECTLY to any aircraft using the aerobatics box. You must yield right of way to THEM. You might be able to share the air during your transition of the area, at THEIR discretion.
Communication is the key. Just listening on a radio is not good enough. They need to hear from you if you are in the area. They are not mind readers. If they are surprised to see you inside their area they have to assume other wings may be there and it could ruin their flying day, and you would be violating the FAR's that state that we must yield right of way to them. Here is a tutorial on how to communicate with the aerobatic pilots.