A sending unit should come with the gauge, use it and not the original one for your stock dash gauge they may be set different. You'll need to drain some coolant before removing the original sending unit, enough to get it below the intake level. Pulling the top rad. hose off may drain enough, be sure to catch it all though.
It looks like you'll need to tap into a + source for the gauge from your ign. on or accessory wiring for the Sunpro gauge. Here's some Sunpro instructions from their website. http://www.sunpro.com/publish/2007/0...lish_16428.pdf
'67 Coupe 289 stroked to 333, AOD, 9" w/3.50:1, PDB, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon