[Christ]

The engine should rev down as soon as you let off the gas pedal... my dad has the same engine in his S-10, and it doesn't float.

The float is probably attributed to input/output sensors, honestly. You should get a datalogging tool and start seeing what they're reading at... compare those readings to OEM specs, and adjust/fix accordingly.

Start by cleaning your TPS, throttle body, and Mass Airflow sensor. Those are the most common things to be dirty.

Also - take a can of ether (starting fluid) or carb cleaner and spray around all the gaskets which are in the intake path... spray down all the vacuum lines, etc... this will check for vacuum leaks, and if the engine stumbles while you're spraying in a certain area, or idle speed increases dramatically for a second, you have a vacuum leak... this will definitely affect your spin down speed.

Check the Idle Air Control Valve as well. If it's sticking (it's an actuator/solenoid) it will allow too much air to pass the throttle plate...