You need to figure out exactly what pulley is on there now for one...
Two is that you need to make sure you are running a stock crank pulley to make sure you are actually going to run the boost number you want.
Third is, get with a tuner and talk to them. You can't just toss a new pulley on without a tune anyway and a good tuner will be able to recommend a correct pulley size based on your mods and desired power level. Non-intercooled should be limited to 6-8 psi anyway.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.