I have a 1968 130 (2-door) 122S that spent 20 years in a barn. It came back to life with ease, but quickly developed serious issues with rust flakes in the tank. It took me weeks of running with large renewable fuel filters to get it cleaned out, followed by removal, water flush with detergent, flush, clean, and finally drying.

I still had a slurry in the fuel for a while and it was clearing, but it was completely undone when the internal fuel pickup line broke off - forcing me to fill up after 4 gallons.

I got a used replacement with only a little sludge from Chris Horn (a.k.a Agent_Stranglove) It also took a while to flush out (almost there) but functions just fine now. I still used a fuel filter just to be safe.

As to your draining, use the draincock on the bottom as sdewolfe suggests, but be prepared to removed the tank and clean it out as soon as possible. It's really pretty easy I've removed them from both wagons and sedans.