In an evening of brainstorming and Canada's favorite fire starter, Crown Royal, we decided that clean pea gravel and a hammock would do the trick. Glad we didn't think about jacking up a truck and tying the fuel tank to a wheel but that seems like a better sober idea.
We tied up the hammock, wrapped it around the tank and spent the next 26 ounces shaking, rolling and sloshing the rock around inside the tank. It took and hour to tip, dig and flip the scale and rocks back out of the tank but she's still holding fuel. A light chain, or several chunks of chain slid between the baffles would have been a lot easier to remove so that's a good idea.
Most radiator shops have the proper tanks and disposal facilities to clean out a fuel tank with chemicals and some offer different products to coat the inside of the tank. The M135 Goat team did the full coating deal and Goose just had the clean out done with equal success so far.
You say the filter is getting plugged up! Is that an in-line filter you added, the screen in the inlet tube at the carb, the screens in the pump container in the tank, the screens on the pump itself....or something else?
Some boys have the train of thought that an inline filter would do the job of cleaning the tank so they keep extras with them and prepare to change often.
Gizmo, one of the last M135's ever built did some field bouncing/ troop hauling this summer and bounced the fuel tank enough that 'more' debris was knocked loose inside the tank and we fought with it all summer. Cleaning the tank right the first time will certainly save a lot of grief down the road so let us know how that cleans up for you.