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!