No. The 1984 is a 14,000 imp full scale gauge. The 6000 imp will not work with the 84.
If you want to use the 6000 imp gauge you need to read the signal from the sensor on the harmonic balancer up front on the engine as opposed to the sensor on the ring gear. You will also need a tach amp (which it sounds like you already have).
Likely not the path of least resistance.
So you’ve checked the connector on the EGR computer. And you’ve checked the red fuse on your OVP relay. And you’ve checked the connector on the back of the gauge. And you’ve checked the firewall plug. Huh sounds like you’ve watched my YouTube video.
You’ve covered all the bases for an 84. I think your gauge is messed up. Best guess at least. I have an 85 300d gauge in my garage. Maybe I can read off the caps for you. The 2 caps you mentioned sound like the clock capacitors. Irrelevant.
The frequency to voltage converter is likely an LM2907 IIRC. Let me walk out and take a look. BRB