That FAQ is a great resource. Displacement is measurement of bore size and stroke. The cam has nothing to do with it.
360= 4.05" bore x 3.5" stroke
390= 4.05" bore x 3.784"

An easy way to check the stroke is to pull the #1 and #4 spark plugs. Insert a dowel into #1 plug hole and bring it up to TDC. Mark the dowel. Now put the dowel in #4 and make a mark in the same fashion. The distance between the two marks is the stroke.
3.5"=352/360
3.784"=390/406/427 (very doubtful it's anything other than a 390)
3.984"=410/428

To make a final determination you'd have to pull a head and measure the bore.