69 Charger. Drained the old gas and it was shellac brown. Put in 5 gals of fresh gas, drained, and it was brown. Do I have a rusty tank?

Pour a little on concrete, let it dry. If there is an area of red rust specs where the gas was, you’ve got a rusty tank. You could pour a little on a paper towel and look for rust residue too after the gas evaporates.
If not rust, it may be varnish that’s dissolving in the fresh gas?

My vote is for bad gas. It doesn`t take long nowadays for gas to go bad and turn to crap.

On a side note I would suggest a new tank. Spectre in Canada was a premium one but there was something about some counterfeit ones being sold

You will be fighting contamination issues until you either have the tank boiled which really doesn’t address contaminants in the fuel line or replace the tank. Changing the fuel line would be worth the piece of mind.