Amp gauge shunt

The final couple of years of A-body ( 75 and 6 I believe ) also had remote shunt amp gauges. I don't have those wiring diagrams.
The problem with doing this to a earlier build is getting the higher amp power supply into the cabin. Unless you route all those wires at the weld splice to the outside of the firewall, You might still have the bulkhead connection melt down.
Drilling the bulkhead connector is only removing the bits of plastic that held those crimped terminals in place so a continuous wire can pass through. This is widely considered necessary regardless which type of gauge is inside.
I suppose you could do like the fleet taxi described above,,, Drill a sperate larger hole through the firewall for another grommet where X number of large gauge high amp wires would pass through.
Edit; After rereading your post, you're considering adding shunts inside, to the back of OEM amp gauge. I suppose that's doable. Some of the above is still for consideration. Good luck with it.