I almost forgot, tipping the head on a verticle mill with a square cornered cutter will also allow you to cut a larger radius with a smaller diameter cutter. You can only use a certain portion for producing a true radius (or very close to it) because it forms an elipse when the head is tilted.
So, for a test of this, tip the head of the mill with a, say, 3/4" endmill in it about 30 degrees. Then cut test part. You'll get a pretty large radius in about the middle 1/2" and then the raduis tightens up quickly around the edges. Somewhere you can find a formula for this process that will produce the desire radius. I've used it many times in the past (20 or more yrs ago).
If you use a ball endmill to cut the rib, tilt the head similarly to above such that the center of the endmill doesn't cut a little swirl down the middle of the rib.
