The alternator is a rather simple modification, particularly if you use a one-wire unit, which I recommend. I prefer Tuff Stuff for alternators and starters. As an aside, your LA360 will accept a 1988 – 2003 “mini Denso” starter used on Magnum 5.2 and 5.9 engines. Tuff Stuff makes a high-torque mini Denso if your engine has higher compression. I have info on part numbers and whatnot at https://poly318.com/poly-318-la-b-rb-slant-6-mopar-starter-motors/ For the one-wire system, you run your properly sized charging cable from the alternator to the battery and a ground wire from the alternator to the engine block. That's it.

Since you're going to run an alternator and to address the potentially smoking gauge in the room, the best way to approach the ammeter situation is to ditch it entirely and install a voltmeter. If the alternator puts out more amps and should the battery draw more amps than the ammeter limit, the gauge will fry. This issue tends to happen when starting the engine after the battery has been drained some due to cranking the engine or using juice with the engine off. You can modify ammeters with a shunt resistor to handle higher amperage, but that's more complicated and for another discussion. You can safely run the ammeter with an alternator if its regulator doesn't surpass the ammeter rating since the gauge will be protected. I don’t know what a factory 1958 ammeter can handle, but I’d be surprised if it’s over 40 amps. You’d be stuck with a 35 amp alternator in this case that might do just fine if your electrical including ignition system is bare bones. The ammeter wiring is rather straightforward. A wire from the alternator output and another wire from battery power at the ignition switch go to one terminal. A wire from the battery goes to the other terminal, most easily picked up at the battery cable connection at the starter solenoid. For a voltmeter, you run a wire from switched 12V (such as off the fuel gauge hot) to one terminal and the other terminal to ground.

To your question about the ignition switch, the answer depends on the type of ignition system you're running. If you're running points or some version of HEI (often called a "ready to run" distributor), you can use the factory switch that has only one ignition terminal. If you're running a Mopar electronic ignition system with its ECU, you need to use a later Mopar switch that has two ignition terminals. I modified a Standard Motor US50 switch and cylinder used on later Mopars to fit in my 1956 Dodge face plate, and you can't tell the difference between it and the factory switch looking at the dash. The reason you need this switch is for a ballast-resistor bypass circuit. Ignition 1 feeds current to the ECU and input side of the ballast resistor, and Ignition 2 feeds current directly to the coil. When the switch is in Run, full battery current flows from Ignition 1 to the ballast resistor and ECU while no current flows through Ignition 2. The current then runs through the ballast resistor before reaching the coil. When the switch turns to Start, Ignition 1 current stops and Ignition 2 full battery current flows directly to the coil. The reasoning here is engine startup. If you use the factory switch with only one ignition terminal with Mopar electronic ignition, the current to the coil at engine startup must pass through the ballast resistor. This routing is less of an issue when the ballast resistor is cold since it isn't dropping voltage much if at all, but if you try to start the engine after driving for a while and the ballast resistor is hot, the voltage can drop enough to make properly firing the engine difficult or impossible. The bypass system feeds full battery voltage to the coil only when the engine is cranking in order to facilitate stronger ignition.

I hope my input helps.

Happy to help. Don't let anyone deter you from using the Mopar electronic ignition with ECU and ballast resistor. It's an excellent system that when coupled with a blue high-rev ECU performs extremely well into 7K RPM.

In the event you don't already know or don’t already have the parts, the ballast resistor you want is a WVE 6R1002 that on the part is marked "R55." It is the correct 0.8 ohms for factory Mopar HP coils and excellent quality. I run a Standard Motor Blue Streak UC12X coil with that ballast resistor, and the resistor voltage output has always matched the coil spec. You can get both parts on Rock Auto amongst other places.

For the ignition switch, you're looking for a Standard Motor US50 or whatever cross references (TRQ brand part number is SWA40904 and is likely made in the same factory). The lock cylinder and key is part number US-12L, although I think (but don't know for sure) that your 1958 cylinder will fit that switch. I'm looking at an image of a 1958 switch on ebay, and it looks like the switch secures with a large threaded nut the same as the later US50 switch to where you might luck out and not need to make any modifications. My 1956 switch secures into the dash face plate with a screw in the side of the switch housing, so I had to file down the US50 knubs and drill and tap a hole in the housing. For reference, here's the US50 switch and the terminal setup you need:   

(smp-us50_w.jpg)

(smp-us50.jpg)

smp-us50_w.jpg (11KB - 23 downloads)

smp-us50.jpg (21KB - 22 downloads)

And for the wiring:

(Poly_318_Mopar_Ignition_Wiring_3.jpg)

Poly_318_Mopar_Ignition_Wiring_3.jpg (45KB - 19 downloads)