While I'm not sure what you mean the by 38th parallel (typo?), perhaps he was thinking along the same lines that I am in making my AH:
Henry Clay in specific and the Whigs in general were for the idea of internal interventions by the government, such as infrastructure projects, in order to keep the American growing, rather than the alternative of territorial expansion offered by the Manifest destiny ideology.
MD was a divisive issue back then, much more than most of us realize 100-150 years later because we're in the timeline where MD won.
If for whatever reason Clay had won, I think that both his foreign-policy initiatives that Andrew Jackson worked so hard to defeat in the 1820s and his feeling that territorial expansion came with too many problems are indicative that he would have accepted a boundary south of the 49th parallel.
In OTL 1845, Buchanan's proposal was rejected by British Envoy Richard Pakenham who once again proposed HBC Governor John Pelly's desired border of the Columbia and Snake Rivers. The British Foreign Secretary, the Earl of Aberdeen, later agreed to the 49th parallel.
So if we assume that Clay's Secretary of State also would have made a similar offer, to be rejected by Pakenham, followed by Pelly's offer, I think Clay would have been inclined to accept it to avoid conflict with Britain but would have felt that the Snake River came too close to the 42nd parallel. Hypothetically he may have proposed the Salmon River as the boundary instead, to ensure that the Oregon Trail was mostly in American territory, plus navigation rights. This is something that Pelly, Pakenham and Aberdeen would all have probably agreed to.
Timeline Δ - Maps of an Alternate History of the World