Several factors to consider:
The petrol tank needs to be in place so that you can judge how the nose of the seat will clear the rear tank cutout and possibly how much top clearance is required.
You need the correct length main springs with the correct seat posts.
The springs need to be plumb both side to side and front to back. You may have to drill new holes in the seat frame and or make new attachment brackets to set the nose back from the tank.
Above all, the seat needs to be level and the springs not leaning either way. The springs can be turned in place to help plumb.
Once the back end is figured, you can fashion the front brackets. Try mockups using cardboard or light aluminum. If you don’t have a proper press, the final brackets can be formed in a vice using a torch to help make the bends.
Every fit up varies slightly.
Last edited by wade.edwards
on Mon May 11, 2020 7:58 pm, edited 1 time in total.