@mayflower1000 you do need to pay tax quarterly, unless you are subject to the 'safe harbor' provisions to be exempt from interest for underpaying.
there is no such thing as a 'Roth Conversion Tax'. it is simply income tax payable.
Your situation is no different than someone who sells real estate or stock at a large profit and has to pay tax on that gain. People pay quarterly taxes 'all the time' because their income is 'lumpy' each quarter and they want to avoid interest penalties for under paying.
Your Roth Conversion just creates a "lumpy' quarter. See Form 2210 as there are ways to be eligible for the 'safe harbor' provisions.
