"since we had paid capital gains taxes on these funds for years that we wouldn't have to pay income tax again"
That's true but your records have to show that.
Mutual funds in recent years do that adjustment for you.
If you purchased the funds decades ago you must have good records.
See IRS Pub 550 Table 4.1 for an example of records you need.
as far as stimulus checks go, once they are in your hands, you get to keep them.
Don't file until you get the next $1,400 stimulus check.
