clean the gas tank

[first truck]

I just got this truck and it sat for 20 yrs, with gas in it of coarse]. The person I bought it from did clean it out before hand but after running a tank of gas through it I must have loosened up the material he sort of left behind. I shook enough out of the tank to tar my driveway. Now what is the best thing to use to clean the rest, Should I just soak and rinse a thousand times or is there something like Zylene or MEK that would work with one shot? I took chunks out that I had to break with a screwdriver to fit threw the filler hole! Thanks