Coolant Leaking problems of the 2003 GMC Sierra 1500

My 2003 GMC Sierra 1500 presently has 67,631 miles on it. With about 46,000 miles on my truck at the end of 2011, I noticed low coolant, but could not find a noticeable leak. I continued to add coolant, but could never find the leak. While having my oil changed in June 2016, the dealer (where vehicle had been purchased and serviced) said coolant had leaked into the oil due to a cracked cylinder head, and estimated approx $3500 to fix it. In Dec 2016, they recommended replacing the engine at $6600. In researching this problem, I found gm service bulletin #06-06-01-019b, 12 June 2007, subject information on gradual coolant loss over time with no evidence of leak found. It discusses a known manufacturing defect with castech heads used in the 5. 3l vortec engine as I have in my truck. Subsequent online searches indicated this was a common problem among such gm engines. I went back to the dealer with the service bulletin and asked what gm would do to assist in the repair of this known manufacturing defect. The dealer said the vehicle wasn't under warranty so they wouldn't do help. I requested assistance through the gm regional rep, and was also told they would do nothing. In March 2017, I wrote a letter to gm customer assistance center requesting assistance. Gm responded with a letter two weeks later stating that for service request number 8-2815264552, they "cannot provide any cost assistance on the engine issue on the vehicle. " further they stated there is no recall or special coverage for the issue. The faulty casting of the castech cylinder head in the 5. 3l vortec engine is a known defect in about 5 years of gm vehicles which gm refuses to acknowledge to customers, but clearly acknowledges through its service bulletin. Gm should provide a recall or at least financial assistance to correct this known defect.