This is what I keep telling people, but on BFSSFG, it seems people insist on spending $50 on a Surly Singulator - which won't work better than a crappy, used and busted derailleur. Some people just aren't happy if they can't spend money.

This wasn't directed to the OP at all.

To the OP: the biggest problem you have is the vertical dropouts. You absolutely must have a chain tensioner, and don't let people sucker you into the magic ratio deal. Magic ratios do not actually work, in the long run. After a couple of weeks of moderate cycling the chain will elongate and get dangerously slack. With magic ratios chains get elongated quickly, because usually they are under too high tension (and that is when the magic ratio is done right - when it isn't, the chain is slack already to begin with).

By the way, now that we know you have a thread-on freewheel, you can replace it with a singlespeed one. Whatever you buy, don't get a Dicta freewheel, because it will be very hard to remove - there is no commercially available remover for them.

Last thing: don't throw that 5-speed threaded freewheel away: those are hard to come by, and you have a Suntour Alpha, which is almost as good as their Winner Pro line - rather good stuff for their vintage.