If you can mod the H & R barrel, why not the Henry, it has a 16 twist which DWs had originally then offered their guns with, two, one at 16 and one at 14, then went to the 14 twist, like the Python and got good accuracy. I own several .358 barreled rifles in 1-14, chambered for 357 Mag and Super Mag cartridges for .358 rifle bullet use. If I was getting 3" at 100 I would slug the barrel to find out what was up with that. My 358 MGP will do .4 " at 2,500 fps with powder coated cast at 100. The MGP is pretty close to a rimless 357 Herrett for use in an AR-15.
this is my 357 Super Mag with 16" barrel on a Cadet action. Kind of what I have in mind for a used Henry.