Not DVKK but try pulling the line off where it comes from the bottom of the tank just above the tranny. Wear gloves and clothes you don't like because it's probably going to end up running down your sleeve and bad gas is like skunk to get out. If you have another piece of fuel line ready you can stick it on the tube and catch the evil gas in something.
Once it's stopped draining remove the nut on the bottom of the tank and the pickup tube/screen (if it has one) and let the last few liters drain from the sump.
If it's so rotten that it won't drain your'e better off to pull the tank because it's going to need boiling out at a rad shop. You can (barely)pull the tank on a pancake bus without pulling the engine if you remove the center and rear mounts and lower it with a jack. A much easier job with the engine out.
Have a look under the bed in the back and see if someones already cut an access hole for the fuel sender, many 74's have had sender issues and sometimes theres a hole near the front of the engine deck covered with duct tape. it could possibly be sucked out with an air powered vaccum through the sender hole, NOT A SHOP VAC!!, BOOM.
Flush it out with lots of methyl hydrate.
Let me know if you need the measurements for the sender hole location.
While your'e in there look at the condition of the hoses on the vent lines, they are often cracked and cause gas fumes with a full tank.
