Checking a ground with your ohm meter while the system is off is not an adequate test. The ground should be tested for voltage drop with the system running hard. On a related note, a continuity, or "beep" test, is even less useful as it usually only indicates 30 ohms or less resistance.
Your high current ground should go directly to the body with all coatings scraped off down to bare metal, the ring terminal should be secured with a bolt, a lock washer and a nut(never self-tapping screws!!), and if you drill a hole the underside of the connection should be sealed to keep oxygen and moisture out. I use 3M rubberized undercoat spray. I avoid factory bolts and instead drill a hole in the floor where I can easily access the underside.
Even on a unibody car you want to look for the most continuous piece of thick metal leading toward the battery. Don't ground to a part joined to the chassis by spot welds or adhesive. Even car manufacturers make this mistake, so don't assume a factory ground is ideal for handling 100+ amps of current draw. Vertical panels often are welded to the pan, and that's one consideration for not grounding your amplifier on a strut bolt.
