moving instruments around in FSX is very easy to do so long as you are trying to take an existing gauge and move it to an existing location.
I am not familiar with the differences between the SP and the N models as far as gauge placement goes, but your just looking to move the tach to say the altimeter location then *all* you gotta do is...
Go to windows explorer, navigate to FSX, go into simobjects, airplanes, c172, panel. When in the panel folder it will be a bunch of pictures of the instrument panels minus the gauges but all the way at the end is a CFG file. right click and select 'open with' pick your favorite word processing program. I use notepad. When you open it you will find all the guts of the C172 panel. Find the gauges you want to move around, When I tested this out I swapped the altimeter and the clock. Was funny seeing a large clock and small alt. To swap them just copy the string of numbers on the right of the gauge name to the gauge you want to swap with. Just a simple copy and paste. Save that file, In my case a notepad file, and when you fire up FSX your changes should take effect. FSX can not be running when you do this cause the program will need to be restarted otherwise.
If its not as easy as rearranging existing gauges then you might just try searching some of the freeware sites for a new panel.
Thanks. I worked it last night. Even figured out what the numbers mean that allowed me to move the Tach to the exact location which is to the left of the yoke right under the turn coordinator. Then did some bitmap editing and moved the hole over too.
The only issue now is that a rectangular label which was to the left of the yoke is partialy blocking the bottom portion of the tach. Not sure what it is called in the cfg file to move it, but will figure it out somehow.