“The problem is that interstates don’t fall under anything in the Constitution’s Section 8 of Article I. So Eisenhower needed to first lead Congress to successfully propose an interstate highway amendment to the Constitution to the states before signing such a bill imo”
That is incorrect The interstate system is what they uses to call a “Post Road”.
Many of theses roads were built or established in colonial times to permit commutation and trade. Some of them still retain their original name such as the Boston Post Road:
http://en.wikipedia.org/wiki/Boston_Post_Road
The power exist is Article 1 Section 8:
“To establish Post Offices and Post Roads;”
What is interesting about this particular post road power is that it is an addition to the original articles of Confederation.
Federal courts early on have broadly interpreted post roads to include almost any road.
Interstates are state roads built to federal interstate standards for moving troops and arms
James Madison based his constitutionally required veto letter for the public works bill on a discussion about canals and roads at the Constitutional Convention. Benjamin Franklin had suggested adding canals (for moving freight) to Clause 7 of Section 8, the clause that gives Congress control of mail roads. But the delegates had rejected the idea as evidenced by the following excerpt from Thomas Jefferson’s writings.
“A proposition was made to them to authorize Congress to open canals [emphasis added], and an amendatory one to empower them to incorporate. But the whole was rejected, and one of the reasons for rejection urged in debate was, that then they would have a power to erect a bank, which would render the great cities, where there were prejudices and jealousies on the subject, adverse to the reception of the Constitution.” —Jefferson’s Opinion on the Constitutionality of a National Bank : 1791.
Regarding constitutionally defensible federal spending projects, given FDR’s popularity he could have led Congress to propose amendments to the Constitution to the states before establishing his spending programs, the states probably ratifying them. Instead, he knocked the Constitution off of its foundations by leading Congress to establish his spending programs without the required consent of the Constitution’s Article V state majority.
Same issue with Eisenhower. He should have taken advantage of his popularity to establish the nation’s interstate highway by first getting the states to appropriately amend the Constitution.