I don't see the need to count the number of pebbles unless you have WAY too much time on your hands. As you're emptying it, you'll know when they're out when it gets quiet. You'll know.
I've cleaned numerous gas tanks and I've used (wonder of wonders) gasoline. I've sloshed a quart around inside and poured it out until the loose chunks stop coming out. Your "rock tumbler" method will probably scour the inside a lot more thoroughly, but I've never had any problems cleaning mine with gas. If I were going to use your method, I'd use water as the liquid medium.
The steam cleaner idea is a good one, too. To dry it, let it sit in the sun for a day or so.
There are several companies (Eastwood is the first one that comes to mind) that sell products for painting/sealing the inside of a tank. You might want to look into that, once you've cleaned it out real well.
I'm gonna have to do the same sort of thing with my Model A, but it's a cowl tank and I'm not tearing it down far enough to remove it. I was able to look inside it the other day with a flashlight and I guess I'm lucky cuz there's just a slight layer of rust. But I'm gonna run gasoline thru it while agitating it to get the majority of the rust out. (I hope.) I'll be draining it thru a glass sediment bowl so I can watch what comes out.
