The 1000 and 1600 numbers you quote are turns/mile (of the speedo cable) for 15/12 and 2/1 ratio, respectfully, rear wheel speedo drive gears. I don't know for sure but I'm going to guess that transmission driven speedos are mechanical (chronometric) and that the 'modern' magnetic speedos, that you refer to, might not be compatible. You can measure the 'ratio' of your transmission drive mechanism yourself by counting the turns of the speedo cable for, say, 10 turns of the rear wheel (tranny in a gear). The ratio of this number to 10 is the ratio of the drive. To work with a 1000 speedo it would have to be 15/12 or 1.25/1, to work with a 1600 it would have to be 2:1, assuming a standard size BSA rear wheel. You can determine the exact turns/mile you require by calculating how many turns your particular wheel makes in a mile, then multiply this number by the drive ratio.
I would have thought a 66 BSA would be rear wheel drive speedo. But it may be a 65 titled as a 66 or some other strange situation.....
The 65 service manual has a nice page listing all of the available ratios based upon wheel size, sprockets, etc. But be careful, a lot of the combinations are difficult to find the correct gears for. There are also gears that are misidentified in people's stock.
If I remember right, the 65 parts book identifies the correct Smiths magnetic speedo to use as well.
I hadn't thought about the drive sprocket affecting the ratio ! I might give the back wheel revolutions/speedo drive revolutions a try just out of curiosity - I had planned to fit a 19/47 set up. The bike is a matching numbers 66 - but who knows when the cases were made ! It did come with a rear wheel speedo drive as well so I've got the option to stick with that instead - it's not a detailed show bike so whatever works is good for me. I fancied the gearbox drive purely to have a shorter cable and less "mess" along the swinging arm. But I've just come out of the garage and looking at the bare frame on a table and the scattering of engine parts on my bench I think it'll be a while before the speedo comes into play !!!