Front wheel drive recumbents not requiring welding are popular in instructables:
I did one myself : http://www.instructables.com/id/simultaneously-pedalling/
These front wheel drive bicycles are easy to build, cheap and do not require welding. However, they have two main problems: steering is affected by pedalling and traction is lost uphill because the weight is shifted to the rear wheel. You may get used to compensate the steering influence of pedalling but it will always be impossible to use the bicycle on a hilly terrain. My rear wheel drive bicycle overcomes these problems while keeping the simplicity of construction of the front wheel drive ones.