Hmm maybe something like Eagle Tree's telemetry system and set an alarm when you near the 400 ft limit. The other option would be something like the zLog which records your altitude.

Take your plane to a nearby mall with a large parking lot. Each 'parking space' is about 10' wide. Park at one end of the lot and prop your plane up against your car, walk about 40 spaces down. According to a google search. The "One World Trade Center" in Long Beach is 397 feet tall. At 350 feet is "Landmark Square" on Ocean Blvd. That should give you a visual reference of how tall 350 and 400 feet is.