What is the absolute lowest I can drop the car with custom springs, while retaining the ability to align the car properly. (I have stock suspension)

Hard to say; esp. since there'* no kit offered, and no one on here has done it (That I'm aware of).

and you will have do deal with the ELC unless you disable it

Why would the ELC be an issue :?: If I lower the front and back, then the car would be level.

because the system is only in the rear, and the sensor will think that the car is too low and costantly be trying to raise the rear of the car

I dont think you are wrong but I just dont see how this is. The sensor will think the car is too low based on what? Since its not based on the level of the car? The only other thing I can think of that could be monitored to keep a stock car level is the compression of the shock. Is the sensor not so much monitoring the level of the car but rathing keeping the shock compression at a "stock" level

ok there is a sensor that attaches to the rear controll arm of the car. this sensor tells the compressor if the car is too low or too high....if the sensor sees that the car is too low it will pump up the rear shocks to keep the car at the proper height, if the car sees that the car is too high it will vent the air out of the compressor to keep the car at the proper height.

if you lower the car the sensor will think that the car is too low and will compensate by rasing the car to the stock height anyway. So you have to dich the compressor and the ELC system

just wondering could you just not move the whole sensor up or down to get it to think the new lower is the old stock

I thought someone posted a link to some edelbrock drop springs that were avail for Bonnevilles