I'm new to this carpc stuff, but the old IR sensor doesn't have to be removed when you solder the new one?
I have mounted mine 90 deg. off compared to yours, along side the big plastic bracket.
I have the 669hb, not sure how much difference is between the two, but on mine I had to cut off half from the riser of the h bracket and the frame risers to make more room. the switch board I had to install it on the back of the lcd board, holes almost match up and used some plastic risers.
I wonder why did you install the mobo on a plastic sheet?