Where in the Constitution does it say that the above is not constrained by the enumerated powers clause?
It doesn't, and in Federalist 41 Madison says so explicitly and vehemently, a statement he repeated in at least one important veto, and in numerous statements on record before the House.
In fact, properly understood, the general welfare clause actually limits power even more than the enumerated powers clause, because it says that in exercising its enumerated powers, Congress may only do so for the general welfare i.e. The welfare of everyone. Thus, it is within Congress's enumerated power to establish a post road, but it may not do so for the sake of Fred Zarguna, or for the sake of some people in Flushing, but for the general welfare only.