Pull the stock sender out, put the aftermarket one in, run the wire/mercury tube through the firewall to wherever you are mounting it and then power the gauge off the fuse box or another power source. Stock sender on the 5.0 is right next to the tstat housing (driver's side). You can also keep your stock sender working by using a tstat housing with a bung and putting the stock sender there. Then just extend the sensor wire to reach and no dead gauge.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.