I'm assuming that your MG has a coolant resevoir and your float is inside of it. And that there is no way to mechanically dampen the float's movements due to vehicle bump and sway.
Is that correct?
If so, then the RC time constant delayed alarm circuit you're considering is a good start on the concept, but ill-applied in this case (as an alarm delay device).
A workable, although more complicated, solution would be a 555 RC timed voltage comparator circuit.
Do you need me to go further with this concept?