In your previous post, someone suggested, and I totally agree, that you spend the money for a new tank. On a 30+ year old tank, you'll find that there is alot of rust and it is going to be labor intensive to get it clean.
The cost to properly clean a tank properly would just about equal the tank cost due to it being readily available. Here is a kit you can use to clean a tank properly, which comes from Eastwood. Also note that you also would need 2 quarts of Muriatic Acid, and 1 quart of Acetone along with this kit.
If you can't handle the new one...get a length of chain, anything from dog leash size to 3/8" the bigger the better tho heavier..put it in the tank and find somewhere you can get it balanced right to shake the cahin around inside. Rail of a triler, chair turned on its side, ..idea is the chain scrapes around inside cleanning off the loose rust.Gallon or two of water in tank helps. Remove the chain and liberaly flush with garden hose. Much easier if you hae a drain plug to get the junk/water out tho. Bit of a battle to get thee water and stuff out when it has to go out either the fill neck or the sender mount area. Let dry or if in a hurry add gas and dump a few times. Have a few sprare in line filters on hand when you run the car. Check that pick up filter sock before you put senderback in...If tank doesn't have any pinholes from rust out with some effort you can clean and reuse...
Now that you have asked twice: At a Fairlane tech site it was suggested that you can remove the tank from the car, remove the fuel pick-up and then use liquid laundry detergent and water. This is their advice, not mine. ::