Sadly you are a victim of pattern panels.
The quality of these are not bad but a certain amount of tweeking is required for a perfect fit.
Technically you just butt the wings up to the front panel and weld. Although before I welded mine up I did a trial fit and had the same issue as you. With the main upright joins welded, this will hold the wings together, then with the bonnet in place you can gently lean on the areas pointed in the photo. This will push down the leading edge of the wings and the front panel enough to give you the clearance all round the bonnet.
The bonnet is slightly higher along the front edge anyway, but not quite as much as yours. I would imagine the bonnet wont even catch over the front lock as there is too much of a gap.
You may be able to lean on these points to improve matters. On mine I welded the top lip together. Which is not original but I felt it would be stronger as it would reduce flex and cracking of the joints. I don't think yours was welded so you may have some give.
The danger is that you may cause the seem to split and crack the paint.
But sadly I don't think you have much alternative.
A good panel beater would be able to make the fit better within an hour or 2 plus a bit of painting.