yeah the frame aint done but you will need some perimeter structure some beams or a lattice work set of structural outer members tying in the outside of the swinger pivot,as well as the motor mounts of course up to the bacbone steering head area .
longer swinger = much higher loads in the normal direction ie basically the lever of the swingarm is longer so the force trying to get the wheels out of line is a good bit more.the frame as stock had quite a bit of integrated structure to fight this force,the oem ugly ass rear frame that looped around so hideously and triangled into the upper rear backbone ,actually served a very important part of this duty,along with the cast alloy mounts.......compounding that issue is now the swinger pivot has huge forces in the more or less vertical plane wheras the stock twin shock lashup had very very little vertical loads at the swinger pivot.
basically if you could have layed the bike frame on its side supported at the contact patch area of the wheels and start stacking weight over the swinger pivot then you can see cleasrly that the frame wants to bend in the middle.same test with the ugly looping rear frame removed ,or just cut loose from wher it ties in at the backbone, would be quite alarming a rubber band is what it would be like in comparison.
you can hopefully see that a longer swingarm would be resulting in more deflection in this "test"
having the overall frame centerarea weaker or more springy than it was originally is well a mistake.. but then you add in the constant up to well over a thousand pounds of totally new vertical forces see sawing up and down at the pivot spindle (stock vertical loads were proly a cupl hundred pounds max if that),whilst it is trying to bend sideways at the same time and you might understand why you need more than just side plates
the single forward mounted shock,along with the longer lever is the culprit directing highly multiplied suspension forces in a vertical plane where there was virtually none before,right smack dab in the middle of your modding