Bob,
Basic timing is based on engine load and RPM. But the basic map is then modified by things like intake charge temp, coolant temp, ambient air pressure, fuel type, etc. The goal is not only to make good power, but to adjust for various conditions on the street, and prevent detonation.
That makes good sense. It would be nice just to pop in a Crane Pro curve and ignition box and keep it simple for a 1965 based road racer.
Found this they have been around for a long time and make this ignition only box.
G4 Lightning - Ignition Management — Link Engine Management Systems - plug-in & wire-in aftermarket ECU's