Here is my two cents.

US economy is good and chucking along, and US corporations are making money.  The stock market has reflected US comanies' earning power and Trump administration pro business policies.  Your investment return should be decent, or even great.  I mean around 8 to 10%.  Now, the question is whether your investment return is high enough to cover your loan expenses.  If I were you, I would borrow HELOC, or even better, borrow a 2nd mortgate with a fixed rate.  This is under the assumption that your investment return can cover the expenses.  I would do anything that I can to delay paying the tax.