I use a 78" belt with a stock sized Metco lower, 4 piece Metco idler and 3.25" upper. A 2lb lower would add 2" and a 3.5 upper would add 0.5". So 80.5" should be in the ball park.
As for plugs, NGK TR7IX is popular if you prefer Iridium, BR7EF for copper. Start the gap at 0.035" and go down from there if you experience spark blow out.
If I got the whipple idler assembly would that effect length? Or would 80.5 be good still?