Re: Straight Edges
Don't discount Garro's $.04 string or the sky, which is still free.
Sight from the outside of the seat tube to the inside of the head tube against the sky, adjust, tack, check and build away. If your miters are in phase it'll sort of want to be straightish anyway. :-)
The string and a true rear wheel take care of the rear triangle. A couple chunks of angle, some clamps and threaded rod help hold things in place if need be.
I built #2 that way and it came out really nice. And I learned a ton by not using anything more complicated than string and sky. It really helped me get a feel for how the frame was going together.
Will Outlaw, Amateur
Build it. Ride the hell out of it.