The spacer is a neoprene washer I bought at Home Depot. A lot of guys on Marlin Owners recommend them and they work for me. It keeps the silicone on the mag tube from turning into a fulcrum if you overtighten the screw.
For a barrel band rifle your assumption is correct. Polish both the mag tube and barrel portion of the barrel band but slightly enlarge the part of the band that goes around the barrel. You can put a bit of RTV here to. The most accurate Marlin I ever had had this feature. I also put a bit of silicone in the forend at the tip around the mag tube to help stabilize the tube at the midsection. This worked very well.
Another tip: Perform the RTV silicone bedding early in the day. After several hours you can trim the excess that squeezes out of the barrel channel. If you wait overnight, it doesn't trim well but it does need to set up for a couple of hours or it is too gooey. Trim the glass bedding in the same manner - let it set a bit before trimming but don't let it fully harden.