Don't cut up the dash. They are getting hard to find that have not been butchered. I just bought a nice one and it took a long time to find it. 90% of them have extra holes. The whole pickup is perfect just the way it is. You might not realize how hard it is to find a '34 pickup like that one. It has a later engine, banjo steering wheel, the hydraulic brakes that are not original and that gauge and all of that is just fine. I would want to keep it just the way Grandpa had it. He was careful not to do anything that could not be unbolted. You don't want to do anything to it that can't be easily undone. Don't drill one hole in that pickup. You don't need to restore it you just need to make it reliable. That doesn't mean updating stuff. That means making sure what's on there is in good working order, especially if it has been sitting. Changee the coolant to a 50/50 mix. Flush the brake fluid out and replace it. Check the back of the master cylinder for leakage and the wheel cylinders. It
s probably a single system and there is no backup if it fails bu the emergency brake. Later vehicles have dual systems. The single systems are perfectly safe if they are maintained. There is no warning light to tell you the fluid is low, you need to keep an eye on it.
Why not get that hydrostatic gauge working. It's the old stuff that makes these vehicles so neat. It makes me happy to look in and see those old fuel gauges with the red liquid. I appreciate the effort that it takes to get them working. Plus, it's fun learning how to do all of old stuff. It tells me this person understands what old cars all are about. There are people on here that will talk you through getting that gauge working. They worked just fine when they were new. The beauty of that dash is its simplicity. You clutter it all with more stuff and lose that clean look. I'm building a '34 pickup too.
Like others said, the stock ammeter is fine. Amperage gauges are the same for 6V and 12V The gauge measure amps not voltage. Voltage is the pressure moving the electrons. One amp is the number of electrons flowing past a given spot in one-second, billions and billions of them. If it was a water hose instead of a wire, amperage would be how much water was flowing in the hose and voltage would be the water pressure making it flow, it's the same concept. The electrons get counted by the ammeter the same no matter how much voltage pressure is moving them along.