Valuing option on a spread

1) How does one value option on the spread between two indexes ? Does one model the spread in itself using a single factor model or does one need to build a multi-factor model to account for the behavior of individual indexes?What are the pros and cons of each?

2) How does one price an option using th forward price curve? Does he simply compute the price change between two points on the curve as a measure of volaitlity and feed it into Black-Schoeles(for simplicity we assume that its a log normal distribution and that we can use BS)?

Thanks