I have an old steel road frame I want to convert to a fixie. It has a standard horizontal dropout on the non-drive side but the drive-side dropout has a flat steel piece filling the rear half of the dropout which effectively turns it into a semi-vertical dropout with no horizontal play. I would like to get rid of this excess metal to turn this into a typical horizontal dropout like the other side. What's the best way? Drill with a cylindrical grinding bit? Jigsaw with some kind of metal-cutting blade? Frame is pretty junky so high-risk strategies are OK.

There's really no way of telling w/o having any idea of your skill/comfort level in metal fabrication, and what tools you have available. For a nice frame I'd probably use a drill bit to cut out the inner end of the dropout slit, use a hacksaw to extend the slot, and then finish it off with a hand file. For a cruder frame I'd simply grab the angle grinder and have at it.

Any chance you could post a picture of the problem?

Most of those were just standard dropouts filled with brass. I would drill a few holes in the filler and then use a hacksaw for the rough removal. Then clean up with a file. The filler has no structural properties. My bet that it's a 80's Schwinn.

Didn't you ask this on another forum? Or didn't you like the answers you got.

Anyway, here's mine again.

Based on a similar application not involving a bike.

Start by drilling a hole of the right diameter in the right place* to form the end of the slot. Use a coping saw, or hack saw to connect the current slot's end to the hole top and bottom, slightly narrower than desired (how close you work depends on your skill and confidence in sawing to a line). Finish by filing the slot to final dimension.

* the location of the hole is critical because it's important that the dropout slots are parallel and at the same height. Otherwise your rear wheel won't be vertical. Don't measure twice, measure 3-4 times and be sure. If you're not sure of the accuracy of your measurements you can file the top of the slot on the lower side using a wheel you know to be true and dished correctly to check until it's exact. later you'll have to remember to always install the wheel on the ground, so gravity ensures that the top of the slot rests on the axle.