How to immaculately clean a gas tank?

I'm attempting to resurrect a 1980s Honda FR500 rototiller with a Honda G200 engine. 
 
I've managed to get it running (poorly), but I'm currently stuck on a problem the internet hasn't been able to solve for me. 
 
While I'm able to get the engine started, the carburetor clogs fairly quickly. When I've opened the carburetor I've found gritty, sand-like particles inside. 
 
These particles are almost certainly residue from the gas tank (which had been full of stale gas for about 20 years). 
 
I tried to clean the gas tank by filling it with some pennies, and fresh gas, and shaking it all around. But that didn't do much. I doubt filling it with gravel and shaking it would clean it very well either. The residue is on there pretty good and it covers maybe 25% of the tank's surface. The gunk is a dark brown/green color and has a slimy appearance, with tendrils. 
 
I've tried acetone, too, but that didn't get all the gunk off. 
 
So my question is: Is there any foolproof way to 100%, immaculately clean the inside of a gas tank besides gravel, nuts and bolts, acetone or vinegar?
 
Thanks for any thoughts you might have. 
 
P.S. I'd like to simply avoid the issue by installing an in-line fuel filter, but the gap between the tank and the carburetor on the G200 engine is so small I'm not sure that's feasible. I'd have to use do a loop-the-loop on of the fuel line to make the filter fit, and I'm assuming that would cause problems?