I have recently acquired an older (late 80's early 90's?) schwinn impact mtb from a guy on craigslist. I bought it with the intention of building it into a solid single speed commuter... but upon closer inspection (when i initially saw it and inspected it, we were in a dimly lit parking garage) what appeared to be horizontal dropouts are actually.... well... i don't know what you'd call them. They have the shape of horizonal dropouts, but most of the horizontal plane is filled in with metal. Essentially, there is only enough room for the axle to fit snuggly. In other words, i guess they're 'technically' horizontal dropouts that are effectively equivalent to vertical dropouts.
My question then, is can i safely file away at this excess metal inside the dropout? I can't really see any reason why i shouldn't, but i thought i should ask just in case this is considered dangerous. Also, if i do decide it's safe, how should i go about doing it?
I really appreciate any feedback... thanks a bunch.