On your progress, looks great!
While your soaking the tank, throw a handful of small nuts in,,,,, and shake the "XXX" out of it.
Will help dislodge the corrosion.
NOTE: Count the nuts before and after..
Are you going to do a sealer coating inside the tank after you get it clean?
On the brass nipple, maybe a "Home Depot", but best to find an old style hardware store.
{In Houston we have "Ace Hardware"}. They carry items that the large places like Home Depot don't...
They will have metal fittings like you need.
My guess; Something like a 1/8" NPT x 1/8" (or 1/4") hose nipple?
After you get thru soldering, I suggest a leak check to ensure that all is sealed.
Plug everything with rubber plugs/etc and do a bubble test with low pressure air.
Ride safe, Ted