To get good data on exhaust pipe length use one of the calculators.
Here's one:
Exhaust Length,exhaust pipe,induction
And another:
http://dairally.net/daihard/chas/Mis...s/DaiPipes.htm
Read ALL about it with this book:
http://www.bentleypublishers.com/aut...ry-1483-2.html
For your engine and the RPM you want, it's gonna be pretty darn long.
If you cut the calculated length in half you'll get somewhat less of the same effect due to friction as the pulse travels back and forth twice, but it helps for compact packaging.
Take a look at a GS500 or Ninja 500/250 to see what they do for the crossover.