I think it was swapped in. I've seen 2wd 1/2 tons, but I don't think they came factory 4wd 1/2 ton.
You are correct. GM did not install a big block in ANY 4x4 rigs from 1973-1980. In 1981-1987, the only way to get a big block in a 4x4 was the K30. No 4x4 Suburban came with a big block from 1973-1991, and no Blazer ever came with a big block.
Martin
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