Engine ?

[Ls6 Ragtop]

Mark,
3963512 blocks were used for LS6 applications with the 4-bolt mains as you say this block has.
G 30 70 is too late for production LS6 cars for 70. It's only production line usage would be for a 71 LS6 Corvette. What if anything is stamped on the front pad? 3994026 heads are over the counter square port cast iron open chamber heads with no production line useage.
If the block has been decked and is unreadable, you'll never know what it originally was.
Chris