dirty gas tank from old gas

[phase_one]

Wasn't sure where to post. I know this board has the most traffic so someone should see and can help. I bought an 92 chevy van for the engine and trans. Of course not all is prestine. The van had been sitting for 4 years. The tank has what used to be gas in it. I use a siphron pump to get the gas out and then dropped it. There is a ton of the tar/varnish in it. How do I get it out? The exterior of the tank is good. I changed the fuel fitler and blew out the fuel lines with my air compresser. I have a new fuel pump and sending unit to install. Just looking for the best way to clean out the gunk. Any help would be good. By the way to tie this to the 67-72 section this will all go in my 70 when all is running.