Originally, I thought to create a rear suspension similar to a Honda Hawk GT (left) -- single monoshock, no linkage. This design allows for the typical 4-5 inches of travel at the rear wheel, and requires a pretty hefty 1050 lb spring, but no linkage -- simple and light!
The swingarm I am using is an Aprilia Pegaso -- it normally has a RHS chain drive, so I simply flipped it for my application. I can conveniently use what was once the linkage tabs on the underside of the swingarm as mounting bosses for the Hawk shock. The 12" ruler is shown as a guide -- the stock Hawk shock is 12" long, and the angle it is set at approximates the angle of the stock frame above. Should use the same 1050 spring, or close, to get the proper travel.
There is, of course, the gaping hole that the original linkage shock passed through. Note that the rod shown will also pass through the rear of the engine case. This got me thinking because if I mount the shock as above, it sits high in the frame -- would like to lower the C of G if possible, and it would be a much "cleaner" solution.
So what I had in mind is similar to the pic at the left. Note that this is a linkage design (hidden under this swingarm), but if I could fabricate a shock mount to weld to the underside of the swingarm, allowing the shock to pass through the hole in the swingarm, the entire design would be stronger -- less material, less weight. However, what would this do to the spring rate required?
If mounted to the underside of the swingarm, the shock will be at a slightly different angle, be about 1" closer (fore and aft) yet almost 3" lower (vertical plane) to the forward swingarm pivot. Conversely, it would be 1" farther and 3" closer to the rear wheel axle. It may make no difference -- some eyeball engineering at work, but I'm gonna get a Physics expert (Ross) to ponder this for me.
Why this is so critical, is that if my new location requires a lighter spring (less than 1050 lbs) I'm OK -- lots are commercially available and reasonably priced. However -- if this new location requires a stiffer spring, heavier than 1300 lbs, I might be pooched. From what I can tell, there is no spring at this length (number of coils) readily available for the Honda Hawk shock I am working with. I learned once the hard way about custom-wound springs and etc -- don't want to go down that road at all! The picture at the left shows the approximate attitude of the shock if mounted according to my idea... we'll see. 
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