Yes a BSA Twenty is a Raleigh Twenty. Raleigh bought up a lot of smaller manufacturers in the mid-century and then released 'badge' engineered' versions of popular models that suited the 'brand'. So some were more sporty, some economy and some more deluxe, going under the names of Sun, Triumph, Rudge, BSA and Phillips, much like VW markets Seat as 'sporty', Audi as 'premium', Sköda as 'economy', VW as 'family'...

(edit) Oops I just realised that you weren't asking but merely stating. Sorry bout that (/edit)

This is probably one of the best places to talk to Twenty heads - many users on here are Twenty-heads including myself, Cheg, Stevegor, V1nce, easy_racer, Jur, sammyboy (sorry those I've missed). Oh and Sheldon pops his head round the door here every once in while too!

Best thing to do with spokes is use an online calculator - you just need to feed in the various dimensions and how many crosses you plan to have in the spokes and it should do the rest.
DT Swiss - the spokemaker has a good one:
http://www.dtswiss.com/SpokesCalc/We...px?language=en

Brakes on chainstay is an inspired idea - will make for a more elegant rear that's for sure.

Links to my R20 site are in the footer plus I have a fledgling wiki set up here as well ; it's somewhat incomplete right now but may have something of interest if only links to other resources.

Have fun with the resto/conversion!