This is a tough one...
While you can build your own frame there is a lot more to consider than welding expertise.
1. When you are done, if you intend to sell them you will need to provide an MSO with each one, otherwise people will have a very difficult time registering them.
2. You will need expensive jigs (especially if you expect to offer different strecth, rise, or rake angles)to maintain straightness and alignments (plus repeatability if you plan on making more than one)
3. Liability insurance will be a big cost
4. Marketing: once you are done and ready to sell them, there are already hundreds of people making frames. If you plan on ust copying an existing frame for yourself that's one thing, but if you plan on selling them you really need to have something new and exciting to offer or be selling them for quite a bit less than one everyone else is.
After all that, if you are still interested in doing it, there aren't really any good books on v-twin frame geometry, but there is a chassis design book that will help with the basics of building a parts bike and has a lot of good info, also this month's Street Chopper, Sept 2003 issue has a good frame geometry article from Atlas.
If you want more info on the book e-mail me prototype@adelphia.net