It wasn't a Detroit. What it would be is the GMC gas 4 stroke V6, 305 cid most likely. This engine block was also used for the Toro-Flow deisel the GMC mad in the '60s. Same block, different heads. They were made in 351 and 478 cid in V6 form, and 637 in V8 form. One thing thet is odd about this engine is that both the intake and exhaust manifold are on the outside of the head, rather than having the intake between the heads.
