Get a speedhut or autometer water temp gauge. Prices vary, the google is your friend, she knows much. Gauge goes in a pod you put on your pillar or in the cluster bezel or whatever. The wire (or mercury line for mechanical) runs from the sensor through the firewall grommet to the gauge. For wiring I generally bug into the radio harness. Power/dimmer/lighting is all right there that way. The sensor just screws into the hole you drill/tap.
For the crossover I pull it when the intake is brand new so no gasket issues. 3 bolts under each end. Put it on a drill press and go slow working with small bits up to 9/16. Then hit it with a 1/2 NPT (I THINK, can't remember for sure) and then clean the hell out of it when done so there are no shavings in your cooling system.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.