Welcome buddy, nice lil stallion ya got there.
I know I'd feel safer with a cage if I was going to do any racing, but adding hills? Sounds like a no brainer to me.
Ya know, I never really associated anything Mustang with anything hill climbing, but anything IS possible.
I had a friend Keith Krumm who mated his '73 Ford Maverick to a monster mudder frame. The floorboard was chin high. It looked like a freakin grasshopper. It did great in the pit, he won trophies, so I'm not knocking the hill climbing aspiration you have.
If you install the cage in preparation/anticipation, what all are you contemplating doing to the suspension before you take it to the hills?
Once you journey into this, you GOTTA post PICS!!
Good luck!
Grego