[fpcopo]

Hi Jon, sorry about your trouble. It is unusual for a big block head to crack by the guide. However the exhaust guide goes into water and they sometimes leak there which is what might have happened to you.
You said you were sending the head off to be welded. Did you verify that it is in fact cracked by the guide?
If it is cracked and you do get it welded, make sure the machinist resurfaces the head and does another valve job. It won't take much if welded right but things move around alot under that much heat. Now, you were very unlucky if the head was NOT cracked but only the guide leaked because usually it just fills the muffler up with water and blows it out when you start it but doesn't hydraulic the cylinder. Was the car steaming when you first ran it? If a guide is leaking it usually steams bad when you run it out of one side.
Believe it or not, you can fix this by getting a 6 pack of stop leak pellets from the GM dealer and dumping them in. We put them in all new engine jobs for just this reason. They won't hurt anything but really stop the leaks. They used to put them in all new GM cars at the factories to stop possible warranty problems. Frank Payne