Ok, next steps taken. I have welded in two strengthening pieces that still need a little cleaning up. Not exactly pretty, but certainly strong, and when filled and painted you won't see the bad welding quality!
I have also taken off all the fasteners that Stan wants and lastly I have managed to take out one shock mount. It is indeed a bus that is stuck through the frame and welded on both sides. The actual shock mount pin is inserted into the bus somehow - I cannot get that out, but as it stands I can simply drill a new hole where I want it, stick the bus back into the frame and weld it again on both sides.
On the other side I will probably just cut the metal around the bus and weld that back where I want it as on that side there is not enough 'double' metal to insert a bus.
But I will await the delivery of my new Hagon shocks that are now on order and being manufactured to my specific bike/weight/location of the mounts. Can't wait for those to arrive!
I have to open my mouth here and give comment with an alternative gusset for your project.
You have considered what would happen if you didn't gusset the frame after relocating the frame rails under the seat and that's a good thing but I have concerns. Your gussets as you have them in now are supporting the load in compression and that would be fine if they were boxed gussets but your's are a simple piece of sheet metal and if they are not perpendicular to the force or for whatever reason happen to develop a small bend, they will fold up. The placement of your new seat rails has created a point at which the load could fold the tubing up at the top point that the new rails marry up to the vertical frame tubes.
I would suggest that you consider extending the original gussets on the inner radius possibly 3" past the lower intersection point of the new frame rails. That way the gussets will be under tension which would be preferred. (For simple engineering explanation, consider that the original gussets did exactly that supporting the radius of the tail section frame). By extending the original gussets past the bottom intersection of your new seat rails you've dissipated the forces over a larger area giving more of the frame an opportunity to support the stress.
The bonus to doing this is that it gives you a better option for your seat cushion by not having to work around the new gussets you just put in and it will look more natural to a standard frame.
Hope you take this as a constructive criticism and not the wrong way.
Thanks Kawboy, I do appreciate the comments and indeed it was an idea that I was going to pursue anyway.... but which I had forgot about until you mentioned it.
I was going to try and use the old 'double-walled' sections, but I don't think that will work. If I go that way I think I'll build new ones on both sides. Then at least I know I have all new, strong metal and exactly where I want it/where it is needed. I was thinking of a half moon shape to extend from 2-3 inches underneath where the new tube meets the old to 2-3 inches above where the new shock mount goes, just before the frame rails bend to form a hoop behind the seat.
The other way suggested by a colleague of mine would be to weld a flat section on top of the new gussets effectively make that a T-section. That way they will never bend, and if I make that piece thick enough, it will also add extra strength across, AND I can use it to attach the seat springs to. That would be far less work (making a double-walled section seems difficult).
In any case both shock mounts will still be attached to the frame using a double-walled piece and I'll extend that as far down as I can.
I lucked out and found a piece of double walled metal in the scrap metal skip. I cut two triangles out and folded and welded up the 'top lip' to give it extra strength. These will hold the original shock mounts, once I have drilled a hole and welded them in.
They they will go onto the frame (roughly where I hold them on the picture) and that will tie everything in nicely and strongly. Then with the additional top flat welded on to the single skin triangles it will hold the shock mounts at the top as well as on the bottom of the new frame bars.
Ok, another weekend of cutting, grinding and welding. I drilled holes in the new double skinned triangular pieces to fit the original shock mounts. I will wait with welding them in until I have the new shocks and have done a trial fit and tack.
These are the shock I am going to get, but 35mm long (rather than 37) and 10mm restricted in their travel, set up for this bike and my particular weight. Once they are here I can do the exact fitting of the new mounts.
I have also determined where the two new front tank mountings need to go and how I will secure the rear of the tank to the frame rails (double clamp with wing nut on rubber pads). Now need to get a pet cock and a fuel level sender.
There is a very small pipe sticking out of the bottom of the end of the tank that I don't know what it does.... I sucked/blowed on it (steady!) and it seems to be blocked... anyone any ideas? The tank is from a Kawasaki Zephyr 1100.
The precise position of the tank has also determined the precise position of the seat and I'm glad to say that when I thought I needed 3 inch seat springs, I was right, so another £5 Ebay purchase was made.
Lastly, the resident fitter at my work had kindly made me a hoop to fit at the rear behind the seat to keep the rear frame rails together. However, they don't seem to be exactly the right shape (as the frame rails bend inwards a little as they come to the top before bending to the rear) so I have to somehow tell him to re-do/adjust tomorrow.