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I'm no structural engineer, but the front end is lacking a massive amount of torsional rigidity which means the lone purple tube in your picture will twist under load/change of direction/cornering. Adding a deck will add some structural integrity, but I suspect the orange lines you show if placed at a different plane/height (from viewing the bike side-on) will add torsionally rigidity, whilst creating improved top-load (in the same way the deep rectangular box section on Bullitt bikes do.
Looks like a great project, jealous of what you're up to, but I have a house to renovate so any bike related projects are on hold...
Thanks for all the input so far
Would things improve if I added the orange lines and possibly take the purple line away?
BB is now higher :)