The LT1 water pump is made for maximum fuel millage not for max. cooling and it pumps at about 1 to 2psi. This is a problem for almost all small blocks made.
Here and example. Two different car magazines built there own twin turbo engines and ran them on the salt flats. The magazine that used the stock water pump blew up and the other magazine that used a performance water pump did not blow up.
GM dose make a high volume water pump for the LT1.
Or you cam get one here.
http://www.jegs.com/i/JEGS/555/51092/10002/-1