Build a New Frame
Hi Vintage Kin,
I also own a vintage kin...Avion to be precise.
OK, so the pipe frames were pretty weak. If I were you, I'd build a completely new one.
Use 6" (150mm) deep channel section, with a minimum thickness of 1/4" (6mm). Put a cross piece made of the same material at least every 48" (120 cm) down the length of it. Do this, and you will be fine.
Use your old one for a template.
I designed a new frame for a 31' Excella that was similar to this, but used 8" deep material. You don't need that thick for a short one like your's.
Best of luck, and don't hesitate to contact me if you want more detail. Be happy to help you. (I'm a structural engineer among other things.....)