I built a pedal tad ,it's 40inches axle nut to axle nut . Not to discourage you any but iv'e been fabin frames for a long time and even with help of machinest friend it was a chalenge. We made a cross member out box beam and used some kids bike head tube and neck steam for parts. The hard part came in milling beam at corect angle to allow wheels to "cant" , then drilling hole in neck so axle was level and 90deg. To centerline.
The cant angle on those fancy greenline tads is 11deg off plumb at 90deg to centerline , so we copyed that . Turned out ok after a few tries and rode well as far as wheels not jerking during turns .
We used an old kitchen chair as seat (works and looks great 2 bucks at thrift) and got hollow core 20inch wheels from northen tool. There are garden cart web sites that have 26inch hollow core wheels as well.
I am curently building a "can am " style tad to motorize , were one sits at normal height above street. I never felt good riding that low around town and was almost crushed a few times , so use a flag and maybe paint it dayglo green or something. Good luck let me know if i can help in any way.
Sorry no photo having posting issue right now.