I may have found half an answer in Holt who contextualizes Republican pro-tariff sentiment with the panic of 1857 and resultant federal budget deficits. Its not clear that this shows manufacturing interests drafting off of anti-slavery, just that the rising Republicans adopted the cause of higher tariffs in response to a crisis.
After the fall in imports caused a fall in revenue Buchanan resorted to deficit financing rather than slashing spending which “allowed the Republicans to… demand a higher tariff that would balance the federal budget and restore jobs to the unemployed by protecting American industry from foreign competition. By the fall of 1858 Republicans throughout the North, but especially in Pennsylvania, New Jersey, New York, and Massachusetts had taken up the cry for tariff revision.”
Frustratingly, he doesn’t cite any primary sources for that section (my only gripe about that whole book). However, in a nearby section he does cite Foner who similarly describes the Republicans becoming strongly anti-tariff in the wake of the 1857 panic. He goes on to quote William Cullen Bryant who, shortly before the 1860 Republican convention, said: “A deeply-laid conspiracy is in operation to pervert the Republican party to the purposes of the owners of coal and iron mines.”
Although Foner seems to be describing non-principled Republicans bending on the tariff issue to achieve political ends, if what Bryant said is correct then maybe some Pennsylvania coal interests were getting their tentacles in there after all?
Perhaps Robert Toombs was a subscriber to the New-York Evening Post?
Foner, Eric. Free Soil, Free Labor, Free Men: The Ideology of the Republican Party before the Civil War (New York: Oxford University Press, 1995), 171-175.
Holt, Michael M. The Political Crisis of the 1850s (New York: Wiley, 1978), 201.