Rich, there are a lot of things you are mentioning that based on my experience are contradicting. First off, if there is a lot of rust, then 2 gallons of Evapo-Rust cannot get to all of the areas to remove rust, --even when the tank is rotated. (10 gallons Yes, ...2 Gallons No!)
2nd, if there are patches that have been soldered on, then likely the protective finish inside (Terne) has been damaged. Matter of fact, if there is
any rust inside, the protective coating has been compromised at least in that area. Do not kid yourself into thinking you can remove the rust and then add gasoline and all will be well without a sealer applied to the bare metal. It just won't happen as it will be rusted again in less than 30 days even when saturated in gasoline because all of the oxygen in modern fuels.
As far as purging ("neutralizing") the tank, any good soap & water will do this. We personally use heated caustic but in reality, I am of the opinion there has been more "He said/She said" stories posted here about tanks blowing up than reality. Sure there have been explosions in the past but my experiences have been that even when we have used plain water to flush out a tank and then I stuck a lit torch into the neck, the most that I have experienced is a slight "poof" sound followed by a small flame that quickly burns out. While it might be startling, it won't be life threatening (unless you have a heart attack
) Think about it, if you truly flush out a tank with water & soap where there is only a slight amount of water-diluted gasoline left, how much vapor can be present? If you want to disagree with me and my opinion, then choose whatever method you deem as prudent as it is your heath that is at risk.
I have said this before but maybe it bears repeating. Opening up a gas tank like we do it really isn't that complicated, ...its just time consuming to do the job in what I view as correctly. We have a large heated vat that we use to aggitate the tank to soften old ineffective sealers and remove gasoline varnishes/vapors. Afterwards we use a steam cleaner to get a good flush of any remaining residue. Next we have experience doing it where we have messed up a tank or three experimenting to see what works best getting it opened up in the least obvious manner, and we have jigs fabricated to ensure when the tank is reassembled that it is welded back correctly ...and not twisted (easy to weld back together twisted or warped), and we do enough tanks where we can efficiently apply enough sealer to thoroughly coat all of the tank's insides. By this I mean, we actually see in many cut-open "restored" tanks where the tank was rotated and the tank sealer reached the top and bottom of the tank however usually the backside of the baffles were missed in the center, and those rusted areas allowed sediment to re-appear in the fuel causing the very issues to resurface.
I guess bottom line is, I think your plan may be a little flawed based on my experiences but on the other hand, it may be right for you and what you are trying to accomplish.
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