Good information Steve. I really like the look and feel of the 50" inch wheel base (+ or -) bicycle frame. They fit my old frame, which is a little over 6' & don't cramp my moving parts.
The Copper Gator frame stretch would have stopped at 50" if everything would have fit, but because I just HAD to make it into a pedal bike, it was stretched to 53" and that was barely enough & remember Butch I also added 4" vertically & cross mounted the fuel tank as well to make enough room for engine and pedal bracket.
Though longer wheelbase does help stability up to a point I don't see the value of going longer than 50" without special circumstances & requirements like I faced on the Simplex. If that's your case the bigger Sportsman or Bonneville frames just might be the ticket and they also feature the 135 mm rear triangle for moped hubs,thick walled DOM tubing & beefier axle drops as well.
Custom fabrication of your own frame is also an option, but not an inexpensive one to do correctly. Culver City's current build using the nice Helmutt frame. It may be long enough to suit you, you might check.
I wanted a pedal frame for the Harley Peashooter build & the Flyer pedal frame just looked right... although I knew it would not be a "kit" build. The Flyer frame will make this project relatively simple. At any rate I went into this build with eyes wide open & it should be great fun figuring it all out!
Rick C.