Hi Erik
Decide if you will weld or braze this frame then consult known experts.
I suggest Spondon Engineering near Derby.
They still make steel frames for Manx Nortons etc.
If you wanted a specialist to make a frame for you, they offer this service.
They made my frame. They sold me tube.
Boss Bob Stevenson
Spondon Engineering
78 Nottingham Road
Spondon
Derbys DE21 7NL
Tel 01332 662 157
https://www.spondonengineering.co.uk/
Kind Regards Roger