| I've done this and fitted/bodged sidewalk cabrio seats into onto normal seat frames.
The back rest covers are smaller than standard due to the space taken up by the roof mechanism. The covers can just about stretch over the standard frames - it was difficult. The cabrio has extra bits such as a leather strap and an extra catch mechanism for locking the seats when the hood is down. You'll either have to keep these or repair the holes. The cabrio headrests are built into the roll hoops so they're no use to you. You'll need to re-use your existing head rests, which you'll need to cut new holes in the covers and the foam pads so they fit to the backs. I was lucky and found matching headrests on eBay for a fiver.
The seat base is also wider and has a different frame fitting. It can be swapped onto the standard frame but it will overhang and still be too wide for the narrower car body. - I put some stitches in it to tuck the extra width. I also applied felt to the edges which come into contact with the interior plastics to stop all the squeaks the leather made by rubbing where it wasn't designed to fit.
In short, it can only sort of be done and only worth it if they're cheap.  | 
