Creating a hybrid from a classic would not be that hard really but, fuel injection may be necessary.
A controller system for all the components involved would be required though.
Really, a person would need a transfer case to mate electric motor to the drivetrain at the end of the transmission. An automatic transmission would be best.
Battery capacity would only need be as much as required to contain the energy recovered from braking from 60mph to 0. Don't forget the vehicle's weight in that consideration.

Mode of operation:

1. Stopping: You hit the brakes only hard enough to light the tail lights. The controller tells the motor to become a generator and starts to charge the batteries while you decelerate. Once the system see braking for 3 seconds, the engine is shut off for the coast to the stop. The rate of deceleration would have to be determined. Once the vehicle is too slow for regenerative braking, the mechanical brakes would be fully applied.
(if you need Vac for the brake booster, perhaps a vacuum reservoir or an aux pump may be required.)

2. Idle: The vehicle has stopped and the engine is not running. A throttle position sensor on the engine would be required in order to control the electric motor. Fuel injection would be a good idea on car anyways.

3. Acceleration: Once a person hits the throttle, the electric motor does the work of coming up to speed again. The gasoline engine is either started by the starter or, by the vehicle's motion. Based on throttle position, the electric motor can bring the vehicle to speed or assist the gasoline engine.

Not a hard concept and an inclined person could likely find all the parts required to do this.
My plan is to eventually do this to a BIG Chrysler (60s Imperial or Newport comes to mind).

I like the idea of a big car getting 40+ mpg in town.