Craig,
Based on your numbers and the numbers and dates listed
here both of your Powells would have been 1969 productions, somewhere after June of that year.
As far as tanks, I have a tried and true method of 1:4 muratic acid to water in a large plastic bucket. I have used it for years. The last Briggs tank I did was a week ago. Soak for a few hours, drain, soak, so you don't poke holes in it with the acid. If the scale is massive, I use the screws and shake it between acid dips.
You can also level the tank in a vice and pour in some caustic or acid solution such as vinegar, but it takes for ever. Downside of acid dipping is that it will quickly flash a surface rust. The faster you dry it, the better off you are. After that, coat it with Caswells. Do not use Creem and the like. This is not a preference, this is experience of having to tear down a freshly painted tank because the coating had failed.
Here's a link to the E Model I restored last year.
http://www.oldminibikes.com/forum/vbrides.php?do=viewride&rideid=6
