You'll need a combo of tools, a thin cut off blade on an angle grinder and a couple flat files and a round file. Measure your axle and compare it to the dropout, divide the difference by two and draw that out on the top and bottom of the dropout.
If the axle is 3/8" and the dropout is 5/16" then remove 1/32 from top and bottom. In that case I would just file it, I'm not against a little hand labor.
If the axle is 1/2" and the dropout is 3/8", then you have to remove 1/16" top and bottom, that's where the thin cut off wheel comes in handy.