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I had the same issue. The FM was designed > 20 years ago when the best known placement for shoulder straps was "straight back". Thus the 8" separation. New learning (FIA) later showed that shoulder mounts should be closer together. The HANS instructions cover that in detail. The seating geometry on our FM (if we apply the 90's learning to the 80's design) should have shoulder mounts more like 2-3" apart. But this is impractical given the space frame design on our cars. The current post location is a intersection point of 3 major members, and to move them would relocate them to an unreinforrced frame member. Moses had designed bar that would bolt in to the existing mounts and provide 2 new ones. Technically (by FIA rules not SCCA) anytime the belt mounts are moved within a chassis, the car needs some testing and blessing from the cars manufacturer. I suppose that means that some engineer will run it through some stress simulation computer model. So, this quickly get too complicated for the amateur.
Putting all the engineering aside, I called HANS. They told me that in my scenaerio (8" separation on a legacy chassis design with no way to move) the best known practice is to cross the belt behind the shoulders. Left strap to right mount, Right strap to left mount. More angle than specification is preferred over "straight back". At the time, they had several NASCAR teams running in that configuration. At first I even questioned this, but I have ran that way with swivel shoulder mounts and the belt are very well aligned over the HANS, tighten very well, and I'm very satisfied with the official recommendation from HANS.
Check the shoulder mounts on the new Pro FM and you'll see they are now conforming to the new FIA standards. We just have a legacy design and need to adapt the best way we can. One day, I'll probably get questioned by a track official and have to explain why my belts are crossed.
I'll conclude by saying that I'm 5'8" and 140#. I'm one of the few "other" FM drivers that have observed an issue with the alignment of belts over the HANS. I should also mention that I use the FIA approved 2" shoulder straps, not the wider 3" shoulder straps. So, I have less tolerance for misalignment than someone with the 3" webbing.