I did something similar in my red Charger. The stock wiring (running through the firewall bulkhead connector) runs the following:

Gauges
Interior Lights
Head/turn/stop lights
Starter Relay

The secondary electrical system, which uses a master relay, fuse panel, and component relays runs the following:

MSD 7AL
MSD 2-step
Electric Fan


It works well and headlights do not dim at an idle (~950-1000 RPM).

If I were to rip out the factory wiring I'd run the headlights off of a relay as well as the heater blower motors as you had mentioned.

The other item that is often overlooked is windshield wipers. I am not sure if your Imperial is the same as others, but all of the high current switching is done inside of the factory switch that must have been designed by Rube Goldberg. The hurdle to overcome would be varying the voltage output to the w/s wiper motor. In my 66 b-body the heater motor is piggy-backed off of the w/s wiper switch which seems insane to me but what do I know.