Hi all. I am working on a design for my first carbon fiber frame. I have some experience on carbon before, so have pretty good idea on making the frame. However, I have very little knowledge on bike parts.

Here what I have so far:
Geometry similar like Specialized Roubaix with some modification. I keep the head tube and seat tube angle 72 degree.
Fork: I may end up buying some generic fork.
Gear rear 8-10, and front single. (don't know brands and sizes yet)
I like the horn style handlebar with time trial gear shifter. (don't know brands)

which BB should I choose? 68x41 or 73x41, threaded or no thread.

Space between rear drop out?

Head tube diameter? I guess depend on steer tube.

Hopefully I can get some advice before making fatal mistake. Thank you.