SeaFoam is a great fuel additive, but won't do boop for resolving existing rust in the tank. If you're getting rust into the carbs, then there's already enough rust in the tank that the tank needs to be cleaned, de-rusted, and treated to repent further rusting.
Then clean the carbs. You can drop the bowls from the underside while the carbs are in the bike. Then carefully remove the floats and valves and clean them.
Since Stev-o sold that bike to you, I'd be surprised there's a great deal of rust in the tank. He's pretty conscientious about things like that.