Personally have no idea I never ran nitro that large (thats what gas is for ) BUT here is a copy and paste from cooper pipes that MAY (or may not) help you out
http://www.cooperpipes.com/

Starting pipe lengths measure down center of header to weld at widest part of the pipe

21-pipe QHP _____________7-8in
45 pipe QHP_____________9 -10.5 in
67 pipe QHP_____________10-11.5 in
90 pipe QHP____________10-12.5 in
QHP-2 Gas pipe __________13.5-14.5in
21 S-pipe ______________7-8.5 in
Cooper Cobra ___________ 13.5 to 15 in.