^^^Now that's a slick idea! I would put the 30 and 90 ohm guts next to each other and see what the difference is. From an electronics standpoint, an ohm difference could mean just a different resistor. This might be even simpler than it looks.
You might be surprised. GM was so good about utilizing the same basis component and making slight changes to satisfy all the brands that you might be able to just plug and play. If I recall, GM went to the 90 ohm set-ups in the mid-to-later '60s. I'm pretty certain that the Chevelles had them.
I would Google or do a little research and find out when GM changed from 30 to 90. I would start making "recon" trips to the wrecking yards and look for a suitable substitute that has a similar size and the "half-sweep" range like your truck's stock gauge. I would also research and see if a 30 ohm system can be changed to a 90 ohm using common Radio Shack components, like maybe a resistor.
Here's a link that might get you pointed in the right direction:
http://www.novas.net/forums/showthread.php?p=90006
Check this out. It's from a Ford truck forum, but look at Post #3. I think it has the answer you need:
http://www.ford-trucks.com/forums/96...ding-unit.html
Google is your friend.........Let us know what you come up with!
Alex