I know the mounting arms are different lengths on a custom compared to a classic but they should bolt on. I bought a sissy bar from a '99 custom for my '03 classic and it fit but I had modded the passenger seat and it wouldn't fit with the shorter mounting arms. Ended up putting it on my buddies '08 classic and it fit with his stock seat.
I have an 06 classic that came with two. One is taller than the other but both are the same size otherwise. I prefer the taller one so if you are interested in the shorter let me know. I can send ya pics and dimensions if you wish.
Taller is mounted to the bike. Both are the exact same dimensions all the way around except the legs on the tall one are 3" longer. The outside dimension on the lower mounting points are 8 9/16 and
its 12" tall overall. Mounting holes are 1 1/4 center to center. I have no idea if one is off a custom and one a classic or what. All I know is my bike is a Classis and both will fit it. If your are interested shoot me a PM and we can figure something out.
looking at your pictures it seems that the classic side mounts may pull the back rest behind the seat a little. Mine mounts an inch farther forward making it impossible to attach the back rest mount. After measuring mine it seems to be the same. So the difference must be in the side mounts. Thanks for your time though.