Just did this today actually. Im adding a kenwood double din along with running new wire for my speakers. I used the original split din and removed the bottom section.
lotta time lotta tools required. Ull wanna dremel with grind wheels. a drill with various bits the key is drill on the line you want to cut then step up each hole until you can snip them with some cheap clippers. Dont saw dont pull it will pop right out; just take your time. Ill have photos tomorrow of the finished piece (seeing this thread made me wish i had taken photos). Looks alright it still needs a trim to cover the stereos cage i might see if my local audio shop can hunt one down.