Rallye Instrument cluster voltage limiter

Rallye Instrument cluster voltage limiter - 68 Barracuda

Traxfish said
Could you explain how exactly I killed them so I know in the future?

Traxfish said
until the temperature, oil pressure, and fuel gauges started going strait to max travel with the ignition on.

Two possiblilities:
The gages got power continuously (approx 12 Volts with engine off, 14 Volts running)
or
The wires from the gages to the sending units got grounded.

here's how the gages and sender work.
Thermal-Electric Gauges (Session 227) from the 1966 Master Technician's Service Conference

And the filmstrip that goes with that, along with a pdf version of the booklet at
Browse MTSC by Model Year – 1966 – MyMopar

No idea why your FSM doesn't have the Rally cluster. I'm pretty sure the Plymouth '68 FSM does. The '67 does not. Dodge '68 I don't know.

First test I suggest is check each gage's resistance.
Gauge Cluster Issues/IVR

If you set up a live test of the IVR on a bench the output voltage will look something like this on a 0 to 8 V scale.
Gauge Cluster Issues/IVR
Scroll up in that thread to see the resistors used to simulate the sending units.

A lot of good info in this thread as well. Redfish knows this stuff.
Bench testing rally gauges

Here's a pdf version of the factory wiring diagram by @1969383S
https://www.forabodiesonly.com/mopar/attachments/68-barracuda-factory-wiring-rev-2-pdf.1715189875/