I couldn't get the picture to download, but there is a really good side by side image of a 518 727 and 500 at this site.

http://www.forbbodiesonly.com/moparforum/threads/your-guide-to-the-727-904-transmission.17287/

The 500 might alleviate some of your clearance issues but create other around the OD unit. As I recall the transmission mount/crossmember location will also need to be moved back.

Don't know if it's an option for you, but when I decided to go with a 46RH instead of a SB 727 on my 37 I ended up buying an empty 46RH case with the OD housing and pan from the local transmission shop I deal with. That let me do the mockup and build mounts and linkage with out the expense (in case it didn't work out) or weight of a complete transmission. I paid $50 for the case and sold it for the same amount when I was done with it.