That red line gauge is sweet, but it sure is expensive. I put an oil pressure gauge in that spot on my Cuda years ago - it was a hand-me-down from one of my friends, and it just happened to fit perfectly in the hole. It's been there ever since.
The one problem you're going to run into is that the dash lights don't shine in that area, so it's very hard to read the gauge at night. Switching to a 4-light rally gauge light bar didn't help much, but I stumbled across a solution a few years ago. I found that they make blue light bulbs that fit into the side marker light holders for the 72 - 80's Mopars. I shaved down one of the rubber side marker light sockets to reduce it's size (you can find new sockets at most auto parts stores), and mounted it at the edge of the light bar, just above my oil pressure gauge with a small sheetmetal strap. It's up and out of the way, you can't see it unless you stick your head down by the floor and look up, and it adds a ton of light over on the far side of the instrument panel. That blue bulb is a close match to the existing light from the light bar, and I found those at Wal-Mart. After seeing how much easier it was to see the gauges with the extra light, I mounted a second bulb on the other end of the light bar. After struggling for years with the poor lighting for the gauges, it's a nice change to be able to see them clearly at night.
