Hand file out these dropout using a flat file to make the 1 mm of space so that you have parallel sides to within in 0.002 inch. Frequently check your progress with an easy to read caliper so you do not over size the space from filing. If you think this minor enlargement will weaken the dropout add a torque resisting bar.

ps. filing some space off of both faces, 0.5 mm from each side, might be better unless one side has much more metal. Then use it.