2000 5.7 GSI Head and Block melter between 3 and 5 cyl

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Help. I thought I only blew a head gasket. I guess I did, but I also melted a channel between 3 and 5 cyls. It appears I need a long block. I only have 400 hrs on the boat and I can't believe this happened. I am looking for a good source and part number etc for a NEW long block. I'd appreciate any help with a cause and where to purchase.

Thanks in advance.

Engine is 5.7 GSI PEFS. 4012002353. 3869059