I built two recumbent trikes mainly from ideas I found here:
http://www.ihpva.org/people/tstrike/home.htm
My trikes were based on the Short Wheel Base Tadpole Style Trike designs on that site, but I used 26in wheels all around. I can post pictures of my trikes if you'd like.
I also used to participate in the "Great Moonbuggy Race" when I was in college, we used to build folding tandem recumbent off road HPVs from scratch. Some info here:
http://moonbuggy.msfc.nasa.gov/