POR15 and several other companies have cleaning/restoration kits on the market. They all generally involve having to remove the tank and using caustic liquids (acid), so I'd suggest having it boiled out at a radiator shop and then restored. If money is a big issue, then you can do it yourself, but it does take considerable time and effort.
My mechanic's shop does do this and his price was really reasonable when you consider your time.
I found out the hard way that boiling in a radiator shop does work but it has it's draw backs. When I was working we had a tank on a diesel suburban boiled and it worked great for awhile......then it started to run very rough and had no power. The mechanic removed the fuel filter and cut it open and it was clogged with specks of black paint....that threw us for awhile til it finally dawned on us that the black paint specks looked like radiator paint. The radiator shop had not rinsed the tank very good or it took awhile of driving for the sloshing fuel to dislodge the remaining specks of paint.
So if you do have it boiled then make darn sure you get it rinsed completely clean.