Rusty Fuel Tank - Fix Or Replace???

[LaraWilde]

I have been restoring my chev for years... obviously it takes a while.
For the past 3 years it has been at a paint shop waiting for an auto electrician... but it is nearly out.
This week when we flushed the tanks and tried to get her running again... I found that the tank behind the seat is filled with a dirty gunk on the bottom and the filler spout is very rusty.
Is there a way I can clean it up or should I order a new one and wait another 3 months to get her on the road?
I would prefer to get her out as quick as possible if I can.