6-1/2" brace height is what HH recommends for 70" bows; expect to reduce brace height by 1/8" for every 2" reduction in bow length. (Those are starting points, of course.)
12 gr/lb is absolutely fine and probably minimizes noise and any hand shock. 10 gr/lb should be okay; arrows from HH seem to be a bit over 9 gr/lb.
My HH is the only longbow I have that can shoot arrows as light as 8 gr/lb (Dacron string, no Fast Flight -- but that's just me; YMMV.) without hand shock.