The GM part number was 14088824 but nothing comes up searching it. You can expand your search to include Caprice, Monte Carlo and Grand Prix with a 4.3. You might have to buy a whole engine to get one, but if it’s considered a core it may be cheap enough. Probably not a lot of demand for one; I’d bet many were pulled for v8 swaps. Failing that, maybe yours could be welded?