After spending three hours last weekend with cardboard and parcel tape mocking up brackets for the shock I came to the conclusion it looked crap and threw it away.
I have decided I need to transfer the load from the shock to the area around the swinging arm pivot, this is fairly solid and has the benefit of being braced by the engine. The problems with this is chain clearance, if I run the tubing down far enough to get a decent mount point then it will hit the chain. The other consideration is that if I want to run the 660 engine then the frame needs to be modified to clear the chain.
The approach I am going to take is to take the back of the frame tubes of and replace with 3mm plate, as part of this I will bring the plate up far enough to provide a 'fillet' between the two frame tubes above the swing arm pivot. So if you imagine the current layout with a 'V' in the frame, in which the side panels fit, when I have finished the 'V' will have its base removed. I want to retain the side panels so will need to make sure the plate does not interfere with this. Once finished I will then have a solid mounting point, high enough to clear the chain, that will allow me to weld a 50mm x 20mm box section strut vertically onto, this will form one side of the shock sub frame. I will repeat this on the other side and then add a cross member to mount the shock on.
Other considerations are where I mount the ignition unit and the battery, I am hoping the battery will fit in the tail piece, but that is another day.
Of to get the steel from the local steel stockist today.