Found some numbers for the stock suspension:
Fronts: 275 lbs/inch.
Rears: 95 lbs/inch--early 750s were 65 lbs/inch.
Higher performance street use and some track time:
In England, from Chris Robinson who provided the normale numbers.
RMR - The 750/101 Specialists, as well as Centerline and Dave Rugh provide performance kits all close to:
Fronts: Around 550 to 600 lbs/inch.
Swaybar: Original is 17 mm: each offers a bar that is one-inch in diameter.
Chris recommends 120 lbs/inch rear springs.
Centerline advises the front kit with the bar rather than just the bar with original springs.