It's doable without going electronic, the drive cable is long enough. You'd have to punch a hole in the bottom gusset at the head tube, the back bone, and through the tank(weld a piece of pipe there).
I had a 2:1 mini speedo for my non-op shovelhead project that I tried and it turns out it's spot on for the America drive ratio. Either that or I got a miss labled one for the harley

Speaking of that project, it's got a 2 piece 3.5 gal fat bob tank, so the speedo drive cable goes right in the middle no problem. Maybe you could look into a different tank/dash set up while you're at it
My mini speedo view:
