Jon,
I have both a 998 at the console and a 997 up front mounted on the big 2.25" Ram Mounts.
Here is the info: RAM-D-111U-C
Overall length: 5.0", 2.25" ball, 11"x3" plate, 3.68" base, holds up to 15 lbs.
One thought for you, I find having the SI at the console allows you to search and mark waypoints, then you can pick them up on the front unit and know where they are. I have had SI up front for two years and used it seldom on my 798, mainly looking at boat docks. The 997 has only been out once since I moved it back up there so I am not sure how much I will use it on SI.
There is lots of good info around on mounting the units. A couple of good sources of info are the Humminbird boards on Yahoo and Bass Boat Central.
Good luck.