ground it to the driver side motor mount with the bolt that goes into your block. dont forget a ground strap from the block to the fram and perhaps the head to the fire wall.
On a stock car the negative cable is attached to a bolt on the engine near the oil filter. The small wire that comes off the cable is grounded to the inner fender near the battery.
1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. Avoid using the water pump or front timing cover bolts for this ground. They tend to have corrosion problems that make for bad grounds.
Picture courtesy of timewarped1972
3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire. The wire goes into the harness at the starter solenoid.