Will this tube safely stretch?

You will be fine in using the seemingly out of range tube. In road cycling, the 700c tube available in department stores specify a range of 700x23-32, a very wide spread, covering almost every 700c tire width available. Back to your situation, you may notice that the tire will not hold the same pressure quite as long, as inner tubes (being slightly porous) do not hold their max intended pressure indefinitely. This will likely not be an issue though, since they will probably only be at 45-60 PSI. You should encounter no problems. On a side note, I can concur that the slime does work quite well!