Super cool build. I love the rim size and wheel/tire combo. Perfect.
The seat base and rear hump look perfect.
I hope it does not offend but I was gonna ask why you want to use SS screws on the sub frame?
I reason I ask is because SS is more brittle that other steel, they can snap a bit easier. Not that they WILL snap, and there are different alloys of SS, some better for this application some worse. I would say that some grade 8.8 bolts would be the best choice, but I am no engineer. Just a thought. It would suck to snap off 2 bolts and crash.
it is easier to ask for forgiveness than permission