Second edit: Turns out, my original grounding locations may have been OK. After some other issues, I figured out that my engine to chassis grounds were in really bad shape. I had the VDO gauges reading incorrectly, the OEM temp gauge, and the car wasn't starting very well. After putting in 4 new engine to chassis ground wires using 6 gauge wire, all is well.
Edit: I found the problem with my gauges and will change this post to reflect the answer. I had been getting incorrect values on my gauges over the past little bit. This was exemplified on my last interstate trip. I have oil pressure and water temp gauges hooked up and an oil temp gauge which is waiting for spring to drop the oil pan. I drove 5 hours to my sis's house on the interstate and in 5th gear at 70mph, I was at about 240 degrees F water temp and 20psi of oil pressure. At a stop light, I was at 0psi of oil pressure and pegged at 250 degrees F water temp. After fiddling around with the grounds, I got them working correctly. Originally I had all three gauges individually grounded (a single 14 gauge wire from the gauge directly to an individual bolt). I used the 4 bolts under the panel under the steering wheel, two of which you can see below:
To fix it, I grounded it to another location, similar to how JimR did in a previous post:
That picture does not show the real end product, although I did ground the gauges to that spot. This is what I did. For my three gauges, there were a total of 6 grounds (3 for gauge lights, 3 for the gauges). For the lights, I wired the three light grounds in tandem with 14 gauge wire (similar to the pic below but that is the power wire).
Now I had 4 ground wires, one from the tandem light ground wire and 3 from the gauges. I merged two of the gauge grounds into one terminal, and did the same with the last remaining gauge ground and the light ground. To each of these I connected a 10-gauge ground which ended in a ring terminal which connect one of the two screws by the gear shifter as pictured above.
Now I am running a constant 40psi of oil pressure on the interstate and my coolant temp gauge sits around 185-195 on the interstate. At idle, I'm about 10psi of oil pressure. I'm much happier!