Since you are familiar with the 350F, I have to ask. My brother had one about the time I was in junior high. He worked a midnight shift and so during the day, I would "borrow" his bike while he slept. But my recollections of the 350F were that it was pretty anemic performance wise. I remember one day when he had caught on to what I was doing telling him that his bike was not very fast as I could not get it over 90. Was the 350F pretty limited from a performance perspective?Almost certainly not, unfortunately. The 350F has a grab rail that goes around the back of the seat, and that's what a 350F rack would mount to. 350 twin doesn't have one. You might be able to make it work, but it won't bolt on.
There really was not a lot of overlap in terms of parts for the 350F and the 350 twins -- very different bikes.