All the places I used to know that did tank work don't anymore, and the few places I've found that will take on the project send them to Canada. It appears nobody does this work in the US anymore. If anyone knows otherwise, we'd all benefit from hearing about it. Maybe there are a few survivors.
The issue with coating an SL tank is the risk of plugging up the many small fittings, damage to the plastic "flower pot" inside the tank, and obstruction of the internal passages in and out of the flower pot. Add to that the whole question about the long-term impact of ethanol on the coating material, and you suddenly have a fair amount of risk that accompanies the cost of coating a tank.
Getting a tank clean is actually pretty easy. I'd do that first, then inspect the inside as best you can. A flashlight and mirror will help, but visibility is pretty poor either way. If it's not rusted, I'd put it back in the car and drive away. If it's rusted, I'd bite the bullet and get a new tank. Check with the Classic Center. Pricing on new tanks is not as bad as you might think.
The first of my cars that I got running was a 280 and it had a very crapped up tank. I removed it as described, flushed it over and over again with water pouring out tons of crap. Eventually it came clean, but visual inspection showed it to be badly rusted inside. Because I had no idea what kind of shape the rest of the car was in, I didn't want to drop $800 on a gas tank just yet. So I did the chain cleaning procedure that I think I learned from this site. Drop a 6" to 12" length of chain (1/4" works well) into the tank, and slosh that around for a while, flush with water, repeat, repeat. The chain dragging around knocks all the loose stuff free, and if nothing else buys you time before you need to replace the tank, which is exactly what I was looking for.
Now that car is running, and confident that I didn't have some other black-hole of a problem. I ordered up a new tank which is now awaiting installation.
I did a similar thing with another one of my cars, except on that one I could see that the tank had been previously coated and was in OK shape. Once I had it running and could assess it's overall condition, I ordered up a new tank for that one too.