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@ChrisNW
It makes me wince when people talk of hammering, punching and pressing great dents in chainstays. There is a good chance that you may ovalise the tube and crack the brazed joint between chainstay and BB shell, even if you're really careful and use the right tools. I'd guess that frames having pressed tubes are usually shaped and stress relieved before assembly so manufactuers don't have that risk.
Are you going to use the cranks as doubles ever again?
If not, file off the inside tabs. You could even file them down so you have about 1.5mm left on them, probably removing 2.5 mm in the process and still be able to use the inside if you move the cranks to a different bike. If you've already got 3mm of air between the tabs and chainstay, that gives you at least 4mm of movement. The maximum shift of the spider you could get is 3.5mm by using a 103mm BB.
Assuming the dimension between centre line of the chainring on the inside and on the outside is 7mm ((half thickness of chainring x 2) + width of spider across faces):
With a 103mm BB, putting chainring on the outside may give you a movement of your chainline from its current position by 3.5 mm to the outside. Is that what you're after? Any less than 3.5mm and you'll have to keep the chainring on the inside.
From your picture it looks like the cranks could take moving in by 3.5mm, but make sure by measuring that there's enough clearance between cranks and BB shell/cups.
Good luck.