Would I be able to mount that stebel horn beneath the heart?
No, not entirely. Even with a larger Kuryakyn V shaped horn cover I used the intent was never to put the entire horn behind the cover. Just wouldn't fit. Instead cut the bellows/sounder part down to size to make it fit behind the cover after separating it from the compressor unit. Then epoxied the sounder to the inside of the horn cover. Mounted the compressor in the hole in front of the battery. And ran a piece of 3/8" fuel line to work as air line between the two. It's as loud as it would be directly connected. The compressor unit is actually better removed from weather this way. Has worked flawlessly for several years now.
I already had the Kuryakyn cover when I did this. At the time I remember guessing that it "probably" wouldn't fit behind the stock cover. I was wrong though. Not long after I did it someone else did the same thing with their stock cover. That heart cover has always looked to me to be a bit bigger than the stock cowbell. So, while I've never tried it, my guess is that it could be made to fit.
Here ya go, just found the old thread where the conversion was detailed. If the bellows fits in the cover it all becomes a pretty simple little task.
http://www.cvoharley.com/smf/index.php?topic=6622.0