for your 50 gallon tank, just add enough MSR mix to cover the bottom few inches or as far up as the rust goes.
First problem with an old tank is really getting out the old gas. Methyl Alcohol will remove old, dried up gas. Commonly sold as HEET, it makes water soluble in gas, but also melts out the dried crud. Slosh it around, it does a good job. Dump it all, dries quickly. Next, visit any good FARM SUPPLY STORE and ask for Stearns Milkstone Remover. Milkstone Remover (MSR) is about 75% phosphoric acid. Typically diluted and used for cleaning dairy equipment. Last gallon I bought was a different brand, labeled "Super Dicoloid". The guys on the counter will know, about $20.
I have a large Rubbermaid container (with a lid). It easily holds eight gallons of water. Add the MSR to the water (NOT the other way around). From that brew, fill the tank to the brim, and leave it somewhere warm for 24 hours. After you pour it back into the tub (I keep it for years), you will be amazed at the results. Want it cleaner? Do it again. The tub is great, you can hang any old bits in for a good cleaning too (after removing dirt an grease). It works for ages and when it loses it's effectiveness, dilute it with more water, and fertilize your lawn...... no stink and it doesn't hurt Chrome. If you get any on your paint, just dilute with water and wash off.
When finished, I rinse out the gas tank with warm water and then shake a mix of HEET and two stroke oil around, to stop any flash rusting. I never use a tank liner. If your tank has a leak, fix it properly.
