If I needed it, I would grab it quick at $700. The 350 extension and original flange/seal are also valuable.
Thanks to Trends data, it is known that about 2,600 ZL2 cars were 350 auto. L48 auto Z11s are about 2,025, Z10s about another 220. So only about 355 L48 auto cars that weren't Z10 or Z11.
Rare part, not easy to find.