I found a couple eBay deals. Snagged a UMA tachometer that should match with the Plasma II+ ignition, and a PMA6000M audio panel.

I chose the PMA6000 since it has the built-in intercom. I also made the decision to go with a combination of steam gauges and G5(s) or similar for now. I already have many of the parts and pieces, and I can always upgrade to a G3X or something at a later time.