The left side of my engine lid [drivers side] where it meets the trunk sits about 10mm too far forward. To deal with it, the PO just mounted the trunk further back. I tried to adjust it yesterday, so that I can also realign the trunk, but no matter what I do I can't get it to fit any better - the hinge on the lid is as far back as it can go, touching the firewall.

Is there a trick to this that I'm missing? My first thought was to trim a few mm off the end of the hinge and make the slot in it bigger so I have more back and forth play to work with.

Even if the brackets were welded back on the firewall in the wrong position I don't see how that would effect the forward and back movement.

If taking a bit of metal off of the hinge will help. Then do so.

These are really old cars.

No No No . . .
Before you do anything like that, look closely at the hinge on the firewall, they tend to crack at the spot weld since they are under constant pressure from the torsion bars.

Unless it's fixed there, it will just get worse. You're only other option is to remove the torsion bars and do a cheap slap-shot fix on the hinge and then use something like a spring style hatch brace.