O/R H-pipe cause MIL?

yes on any mustang.
O/R in general will trigger the CEL light on your mustang. You get a pair of MILS to turn off the annoying light or you could have a tuner disable the light.

best place to get mils for under $60-80 would be ebay or a mustang classified section.

American muscle is out of stock, but they might have them when you need them: http://www.americanmuscle.com/stmilel.html