Oh, you got it EASY, my friend! That room is perfect for a nice simple install that will suit you perfectly for 2-channel music, 7 channel (or more) HT, AND multichannel high-res music (and/or video) with an asterisk, sorry.
Here's a set of speakers I saw recently on CL. 7.2 just like you'll be looking at. The two biggies are 15" passive subs. The weird looking angled ones are your side wall dipoles. The four clones are the fronts (where you'll be using your existing towers) and the surround backs. The one with the two tweeters between the mid-woofers is the center that will go approximately where your HK3490 is right now.
* You might want to consider additional routing of the existing surround back speaker wires down to wall plates near floor level so you can plug in two towers instead of bookshelves for multichannel music (including 5- or 7-channel "stereo", don't laugh: even though it's a DSP, it is very lightly processed and sound fantastic!). With just the least bother, you set it up such that either swivel mounts up high OR floor standing towers or even stand mounted bookshelves can be used as your surround backs for HT and your rears for multichannel music. I'd kill for a room like yours.