gunked up fuel tank. HELP!!

[rburnett3003]

I'm having trouble with a '77 300D. It's getting air in the fuel lines. First I replaced all the lines, then I replaced the manual primer pump. Then I figured it was the tank strainer, so I replaced it. Still having trouble. I have tried clearing the lines by blowing air through them back to the tank. It seems to work, but only for a couple of miles of driving. I took out the tank and sprayed some degreaser in it, but it still seemed pretty nasty in there and the car has the same symptoms. I still think it"s the fuel tank, because it doesn't seem to vent right and the return line is clogging up. It unclogs if I use a bike pump or compressor and shoot air through it, but then clogs right back up after running for a couple of minutes. I've been working on it so much that i'm really getting frustrated and going broke. My question is. Should I just buy a boat Fuel Tank and put it in the trunk, is there an additive that will absolutely work, or is this a rare enough occurence that I should just take my chances on a replacement tank at a junk yard? I really need a one time fix and to be as cheap as possible. I want to keep the car, but my girlfriend is flipping out about the time and money I'm spending. I'm assuming a new tank is pretty expensive and i'm trying to do this on the cheap. Please help!