Are the caps going to be Helmholtz resonators?
Nope, I ain't doing the math for that. These are just a 4" OD, 8" long, glasspacks. To go with the 4" OD, 15" long, mid-muffler glasspacks.
That said, I am tempted to split the case on the 8" units and make them an oval or case style muffler (like a Borla or Magnaflow).