IME due to flat/riser bars having a lot less reach than drop bars, you'll want a slightly longer stem (assuming same frame geo).
I'd try the 90mm stem if that's what you've got, you may want to go longer. 66cm bars are fairly narrow relatively, so likely you'll need 100-110mm.
I have no idea how to size a stem for flat bars. Care to make a ballpark guess?
I currently ride 40-42cm short reach drop bars with a 90mm stem.
CP will have nearly same stack/reach, and 66cm ritchey 10° bars.
I'm thinking shorter stem to account for the much wider contact points, though I may chop the bars a little if the width is too much.