Riko,
The fuel system can not be contaminated by the water through washing the engine. However, the ignition system may now require the removal and cleaning/replacing of the opti-spark to get it back to 100%. Yes, it will dry out, but the moisture will corrode the contact points.
Imagine what would happen if you took the dist. cap off your 74 and sprayed water straight inside. Then you put the cap back on and tried to start it. It you let it dry out it would eventually start, and sputter until it dried all the way out. That is exactly what has happened to your opti-spark unit. Hoever, it is not very easy to pull the cap off the opti-opti spark to help it dry out faster.
There is one thing you can do yourself today. Pull the coil wire out of the opti-spark. Look inside the terminal end on the coil side. Write back and tell me how crusty it is. Go ahead and clean that out. Clean the tip of the coil plug. Snap it back on and see if it does not clear right up. If that does the trick, the opti-spark may be fine. Where the coil wire snaps into the coil is the other weak link of the LT-1 design. Water/moisture gets up inside the coil wire boot and causes the engine to run terrible. The autoparts stores sell an gel that you can dab between the coil and wire to keep the moisture out.
If cleaning the coil and coil wire does not do it, don't be surprised if the Vette Dr. suggest replacing the whole unit opti-spark unit.