I've restored 2 Olympics, a '69 12/3 and a '67 Super Olympic. When I cleaned the tanks, I drilled a hole in the bellypans and just pressure washed the heck out of them. Once they were good and clean, I welded a bung onto them, so I could clean them easily in the future. A buddy of mine restored a '70 335 E and did his tank a different way. He drilled a hole in the belly pan for a bung, and also created an opening in the top of the tank. He filled the tank with water, and used an angle grinder to cut a 6" square hole around the filler tube, completely removing the section with the filler tube attached. Once he cleaned out the tank, he welded an 8" square plate (with a hole for the filler tube) onto the portion of the tank that had the filler tube on it. Then he sealed the 8" section over the 6" hole, and used large self tapping screws to hold it down. I cannot remember what he sealed it with (may have been a square chunk of inner tube), but it never leaked at all.
The '71 Nordic had a removable plastic tank, as did all the older 18" chassis models and the '71 and newer Olympics. Those were easy to clean in comparison.
Hope this helps! Good luck.