How long do you have to pay the IRS an overpayment you received after you amend the tax return the overpayment is so large I will not be able to pay when I mail the amendment?

You should get the amended 1040X completed and contact your local IRS and ask them if it possible for you to make some installment payments.

You can find some information about this by going to the IRS gov website and use the search box for Online Payment Agreement Application or on the left side of the page scroll down to IRS RESOURCES and choose Online Payment Agreement Application