If the time for payment for any amount of tax has been extended under this section, interest payable under section
6601 on any unpaid portion of such amount shall be paid annually at the same time as, and as part of, each installment payment of the tax. Interest, on that part of a deficiency prorated under this section to any installment the date for payment of which has not arrived, for the period before the date fixed for the last installment preceding the assessment of the deficiency, shall be paid upon notice and demand from the Secretary.