Engine Died While Driving

As title says, was out in the 70 Dart and the motor cut out.

Quick facts:

- Original Slant 6/904 column shift car
- Now a 408/904 B&M floor shift
- Fuel in carb and electric fuel pump still works, so it's not a fuel problem
- No power to anything run off of ignition switch ie: key buzzer, ignition, radio, lights, dome lights, etc
- Switched out turn signal switch a few days ago
- Disconnected and reconnected the Starter and Ignition Switch Connector as it made it easier to fish signal switch wires through steering column
- Alternator (swapped from my slant 6, likely underpowered) shows constantly pinned to charge when throttle is applied
- Ignition system consists of MSD 6AL, MSD Coil, MSD wires
- Aftermarket radio and (1) 6x9 speakers which recently started to have issues ie: only works when brake is applied under 20 mph or so - not sure if related but thought I'd bring it up
- Holley Blue Electric fuel pump run off manual switch straight off of battery
- Electric Fans running straight off of battery

I plan to get under the dash and disconnect, reconnect Starter and Ignition Switch Connector since that is what I touched last, but would a loose connection cause my motor to die?

My first thought was my alternator, but before I go buy a replacement (looking at Powermaster 7509 - thoughts?), figured I'd ask the electrical gurus ( @67Dart273 & @TrailBeast come to mind) on here for some input.

Also wondered if it was my neutral safety switch, but the fact that I have zero power to anything rules that out...

Hope I provided enough info, if not please let me know what else y'all might need...

Thanks in advance!

Wow, lots to take in there, and these kinds of problems can seem overwhelming but don't let it swamp ya.

I have a way of keeping it simple, and probably won't give you a ton of "do this and check that" items but more eliminate one or two things at a time.
Full charging issues usually come from a large draw somewhere, like a short or a battery that won't take the amps the alternator puts out.
Have you disconnected the bulkhead connector and inspected it for heated or melted contacts?
This is a prime source of multiple electrical issue's

You might also pull all the fuses and see if the problem persists.
This will disable pretty much all the accesories, but may give a clue where the problems are coming from.
If some problems stop with the fuses out, then putting them back one at a time a testing again could point out more specifically where the problem circuit is.

Let us know your results.