torque tubes...

I used to stripe them with a crayon and cut within an inch of where it went from burn to melt.
But so did Fred Flintstone.:-D
Go to Wallace Calculators and input your engine information.....it will give you header primary and collector optimum length and diameter for free.
http://www.wallaceracing.com/Calculators.htm

I just realized....that doesn't give you collector length.
Pipe max or any good header program will , but those aren't free.
Ball park is going to be a 12 " extension I'd say......to really tweak every ounce of power from something like this requires a lot of data and will change with the weather , so don't worry about perfection.
Another rule of thumb......short for high rpm and h.p. , longer for more torque.