Oh yeah, theres plenty of room behind there. I had my dremel with the cable attachment on it and a cutting wheel. later i just barely missed the clearance on the passenger side so I notched it out with a power drill. Lots of room for activities. If i were to do it again, and I still might do this to clean it up, I want to run "half" a hole saw on itand once it breaks through the metal, remove those pieces. And remember, prime and paint the area after you cut, no sense in giving rust free access.
Harness is the Morimoto one, and yes, match the stock bulb size, so H11. You do not need a canbus harness for the 14 and older. The make a single harness ( 9006 socket ) and then send a adapter pigtail with it to convert it to the the OEM bulb. You also will only plug the OEM wiring into one side of the harness, the other plug I tucked up and taped real nice with electrical tape so the connection isnt exposed.