Location
I belive you want the pressure take off near the fattest part of the pipe, as to lessen the exhaust pulses from the reciprocating piston.
If you locate it near the header - you will be dealing with variations in throttle output, so that the low end and high end pressures will be so different as to cause tuning problems.
Locating it in the fatter part of the pipe, lessens the pressure variations
and stabilizes the pressure flow.