If you look at the picture posted by CigarCritic in reply 8 of this thread, you'll see the difference in hole patterns for '09 and newer models, and '08 and older models:
The "spacer" referenced previously is shown in the pictures posted by TexasFatBoy and you can locate the part no. for this in his thread, reply 2 in the above thread.
If you want to mount the tour pak directly to the tour pak rack you've ordered, you'll need the spacer--it levels the bottom of the tour pak for mounting to the rack. And you'll need to drill four holes in the bottom of your tour pak to match the holes in the rack. Although TFB's reply is for a chopped tour pak, the chopped tour pak retains the heavily molded bottom of '05 and older King tour paks. '06 and newer tour paks have an essentially flat bottom so the spacer is not needed.
Also, if you choose to replace the backrest pad with a newer version, you can, but the mounting holes in the lid will cause the newer backrest pad to be higher than it would if you used a '97 or newer lid/tour pak. You can drill new holes---same pattern just 3/4" or so lower than the OEM holes--for a better fit.
Good luck with your project.