If you have to do a 5.1...
Keep the L/R sbd where they are. Bleed them into mono for the Center with a slight high pass and make sure they are barely noticeable but are there only for fill. Anything more than ambient(R5 setting in PToolsHD, L7.2 on a Neve) will reduce your image where it will lose it's spacial presence.
After syncing the aud to the sbd (This step is critical or else the entire 5.1 product will be very harsh for listening)...send the audience mixed with a hint of the sbd to the rears, respectively. This level should not exceed 5db that the R5 setting that you have for the Center or again, too harsh on the listening experience.
Surround Sound is a gem to work with. But it is not very forgiving. 5.1 is horrid for audio only. As you know, audio is in essence film's marrionette. 5.1 is a cheapened version of 7.2 or even 11.2 or another friendlier format for the perfect spacial audio element. There are many ways of creating your own surround environment, not one is preferred over the other within the audiophile aficionados, but listen for yourself on what you like. The biggest mistake I find within studios that master for surround is that they don't listen in it's entirety the surround product they release. Many, many, many releases out there are just too harsh to listen to because the producer didn't take into consideration that you would be sitting there for an hour or two listening to a program - given that they only listened to a few minute snippets as he/she was mastering. Something may sound incredible for one song, but what ood is it if you can only sit through one or two of them before your ears are exhausted. Another big mistake is that a program is "over-mastered". You don't really need that much alternate directional sound from stereo to give a listener a 'surrounded' feeling. A slight, minor, barely noticeable sound from a third source behind you is tons better than getting up from the listening experience dizzy from an overdone "surround".
Have fun!