The marine architect firm of Sparkman & Stephens is still living.
S&S designed the T30 among many other boats.
There are some design/build "type plans" on their website if you dig deep enough.
Yacht Design - Type Plans
I didn't see the rudder build details you are looking for. You can contact them by phone; they may sell you the original rudder build plans - for a price.
Generally though, the rudder on the T30 is a skeg hung rudder as opposed to a spade rudder which has no skeg to protect it from submerged objects.
Rudders are almost always foam core filled that are attached to the shaft by a series of horizontal SS straps that are welded to the rudder shaft. The rudder is built over these straps once securely welded.
Sounds like your "straps" rusted out at the welds allowing the rudder to fall completely off the shaft. You will need a good welder if not a new rudder shaft.