Steve....You are not far from getting it right. The seat looks to be relatively new, so is this on an original pan or a pattern part? The cut of the lower edge looks different, and again the seat looks to be low at the back and also positioned just a bit too far to the rear. Check other bikes on the internet for the appearance of the gap between the lower seat edge and the frame tube. The seat brackets look OK and the correct parts fitted correctly. Swapping the rears side to side is also worth a try. The front will only fit one way because of the three hole fixing. On my bike the front seat bracket was a flat plate with three bolts to the pan, but with two extensions forward to mount on that vacant lug which is used when a single seat sprung saddle is fitted. This lug sometimes has a grease nipple, hence the threaded centre hole on some frames. This bracket looked to be a homespun item. I preferred it as it allowed easy removal of the seat without disturbing the stubborn stud holding the frame together. Your front looks like the genuine part, but if the seat and bracket are pattern parts , here comes another source of error..... all the mounting holes in the wrong place.
Mudguards are relatively expensive, I would be inclined to stick with the mudguard you have, mount the fuel tank, position the seat where you think it looks right, then make a new front seat mount to preserve the ( possible) original bracket for the time being.
Looks to be that those rear stays are very wrong, so you have nothing to loose (except the nice finish) by bending them back to normal shape and extending them as necessary to get everything looking right My feeling is that they are not supporting the mudguard high enough. Once it is set up to look right, that no longer attracts the adverse attention from the purists and you have done the best with what you have.
Brett... Thanks for your consideration of the situation, too right its a big can!
duTch...Sure looks strange, as you say hard to tell so measurements from a known good 'un are what we need.
Swarfy.