I know I can use hydrochloric acid to remove the rust.

Then, one could use a baking soda solution to wash out / neutralize the acid.

Then what ? Can you spray a bunch of WD-40 around in there to coat it and keep it
from rusting again until the car is finished ? If not WD-40, then what ?

ATF is cheaper than WD-40.

look online for some stuff called red kote,i might have it spelled wrong?

I've used Evaporust with excellent results. I pour a gallon of it in. I make a small plate to replace the fuel sender/pickup and used the stock lock ring and gasket to seal it up. I would agitate the tank and rotate it to different positions so it would wet different sections of the metal. Even used it on a '70 Duster that hadn't had fresh fuel since the 1980's and had dried out leaving a residue on the bottom about an inch thick. I just added some dime sized crushed granite and agitated it every few hours with the help of my wife. Took a day and a half but it looked like new when done. Best thing is Evaporust is biodegradable and doesn't eat the base metal.

As for keeping rust away if you aren't going to put it in use right away, you can use some atf or other oil to coat it. You can also find a few different tank sealers out there as well but I don't think they're really necessary. The critical thing is to dry the tank completely and quickly after you rinse it out. A hair dryer/heat gun will get the job done quickly and then seal it back up until you're ready to use it.

OK thanks.
I have read about the coatings/sealers. But I read both good and bad.
Just keeping rust away would be my biggest concern after I clean it.

can you buy a new tank ?

Over 25 years ago the tank in my 65 was terrible.Had rust plus crud from gas that sat for like 6 years.Used the POR15 kit.Three part setup.Changed sender like 8 years ago.Tank still better than new inside.The sealer that comes with the kit is super.Just read instructions and its great.I did let mine sit in the hot sun a few days between treatments.Rocky