So I did a ton of thinking through that, JBOT. Did a million Helmholtz calculations, didn't get a conclusive answer (pipes aren't typical Helmholtz resonators, calculations require a lot of assumptions).
In short, I just ran it through the sim. All three of these are new sims, with only one variable changed: pipe length. I added 70mm to both, but one right at the header, and one right at the belly. Frits is correct, at least for this engine. Same tuned length change makes a bigger difference if you do it at the header rather than by moving the end cone.