Being flexible with a half-link and your chainring/cog choice will help. I would pick an arrangement that puts the axle toward the front of the dropout (but with full contact for the axle nuts) so that you have a bit of room to move the wheel back as the chain inevitably wears.

My Diamondback came to me with partially-filled dropouts like this, although I don't think they were this bad. The idea, as I understand it, was to force the rear axle into a particular spot in relation to the rear derailleur for more consistent indexed-shifting performance. (Some bikes had plastic stops installed in regular horizontal dropouts to accomplish the same thing.) It's why the industry went to vertical dropouts not long after.

In order to file the droputs to their full potential , I propped up the frame on its rear so that the dropout slots were vertical, then used a round file and oil. That let me just file downward. I checked my work frequently with a spare 10mm axle to make sure I was opening it wide enough, but maintaining a straight line along the top of the dropouts. Hope this helps.