Accidentally purchased an 850T "H" pipe for my 850T3 FB. The pipe for the T is too wide at the entrance and forces the LH header into the sidestand mount as you mentioned. So I guess the answer is that a floorboard bike needs a narrower "H" pipe than one with rails.
Interesting, thanks! The MG Cycle description of the head pipes describes them as having the correct curvature for bikes with the police-style side-stand, which they seem to have. However, the Mistral h-pipe definitely pushes out the right-hand-side muffler a bit, so that the mufflers aren't exactly centered. My impression is that the h-pipe could be narrowed by 0.5-1.0 inches, roughly. That said, I did get both mufflers to attach to the frame at the stock hanger location. But they are slightly off-center when looked at from the front of the bike. And the mufflers have a slight "rise" going to the rear of the bike.
One other problem I had, in addition to or actually in combination with the side-stand bracket issue, was that I had also replaced the stock engine-to-frame bolts with MG Cycle's stainless steel bolts, which are threaded rods with nuts at both ends. The attraction was that I would be able to remove either side's frame rail in the future without knocking out the entire bolt. The problem was that the rod is pretty long, and if installed evenly it would further restrict the placement of the pipes, both at the front and the rear of the engine.