I just finished doing this, except I started from scratch except for a wheel I had with an 11-28 cassette. I just used the bb size recommended with the single-speed crank, although I was concerned that it would be too narrow putting the chain at an angle with the 11 tooth cog. But eyeballing it the chain is straight on the middle cogs and not seriously slanted to the extremes. I haven't put any kind of guide or limiter on the chain at the crank ring.

So far so good though I've only got a couple of commutes and haven't hammered it at all. I don't think there's going to be a problem. Maybe extra wear on the chain. I don't see why you couldn't use one or two chainring spacers, taking two rings off, arranged to get the chain line you want.