5V conversion help needed

So the backboard in your example is still light colored, flat, quite nice considering its age. Others will find them darkened, warped, and extremely brittle.
If you'll go back and look at it long enough you might finger out none of that was necessary. All anyone needs to do is isolate this 3 post/2 part gauge from chassis ground. Clip the 12 volt wire from the harness connector and route it directly to their supplemental limiter/regulator. Now the copper trace is dead. The stud is no more than gauge support.
No hacksawing, no opening the gauge to bend the little thingy, nothing more than the piece of tape on the back. One could remove that little slither of metal captured there but... that will leave the board a little loose. Again, not necessary. Just cover it with tape or otherwise block it from the housing contact.
The internal limiter is just like any other electrical component. It cannot do anything good or bad if it doesn't have a complete current path (+) -> (-). If the fuel gauge part of it is good it will function just like its 2 post thermal gauge neighbors. But wait... If we cut away the 12 volt going there already, Why do we need the piece of tape? The other current ( 5 volts ) coming in from anywhere can back feed through the original limiter. If grounded it would act as an additional, very faulty, gauge of sorts. So the little piece of tape equates to disconnecting a sender in this current path.
There was a 8 year period when owners would send their gauge(s) to me for service and I would open all of them. For this type gauge I would completely remove the limiter. 95% of the fuel gauges needed the thermal gauge part renewed anyway. That's relative to their duty. The work higher/hotter than their neighbors. Deleting the limiter just for easier reassembly.
Hell I even offered gauge exchange for some models. I aint seen it all ( this thread proved that LOL ). After 8 years a few hundred various gauges, I decided I had seen enough.