Doesn't look too bad. I'm sure you could back flush it and get a bit more life from it.
if you are taking the tank off, Frost do a really good DIY tank sealing kit. Three stage process, degrease and de gunk, rust treatment and final sealing coat. its a bit messy but the results were very good. Worth looking at as a option. I did mine on my 280ZX and very pleased with it.
Huw