I have the Marlin Crossmember with a single case, but I have the two piece driveshaft so it isn't a problem. I don't know if it would fit otherwise.
I thought I heard that Marlin was going to be selling Budbuilt stuff and they have a setup for a dual crossmember that works with spacers for the single applicatoin. They also have single applications.
www.budbuilt.com
You can buy a budbuilt crossmember through Marlin.