The spline bit of the elbow / horn also has a taper so that is part of why they are stuck. As mentioned, a good twisting will get them moving. I think the tapered bit might be steel, so removing the brake bits bits and warming the aluminium bracket with a hairdryer might free them (aluminium will expand more).
If you look at the photo up there ^ you can also see that the clamps are offset. The whole clamp can be swapped LH / RH to reverse the offset and move the brakes inboard / outboard by a larger amount. Be warned – swapping the clamps also means starting from scratch with all the elbow / booster settings and probably deciding you preferred it in the original setting anyway 🙂
I really don’t miss using those brakes…….