I don't know anything about your welder or your welding skills, so I can't comment on your plan other than to say welding thin walled bicycle tubing without cooking the tubing is a learned skill that takes lots of practice.

As far as your gearing idea is concerned, IMHO it's a worthwhile challenge. This youtube video shows a derailleur equipped bike with an added crank similar to what I think you're considering.

http://www.youtube.com/watch?v=FgIL6eHHgZU