You basically have 4 choices.

1) You can buy a Horizontal Dropout and have a local metal shop cut and weld it into your frame. Prob cost you $30 (surly) plus labor.
2) You can buy an eccentric hub. (White industies makes them $130 plus labor and $$ of building a wheel)
3) Chain tensioner ($varies) Surly Singulator about $50. This is probably hte simplest install.
4) adjusting chain length (using half links to get closest) This is cheapest option but you'll have to get lucky to get an exact fit.
good luck.