hi i have a howard hill big five what should the brace height be on a bamboo long bow like this and should my arrows be a couple inches longer than what i shoot out of my recurve

Start with a brace height of about 6 1/2 inches.  May need to increase or decrease it a little.  No reason to shoot longer arrows out of the longbow.

would 12 grains per lbs be all right to shoot out of this bow  as far as arrows goes

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.