That was my exact plan when I got Ken's original reamer from him.
The trouble is, the 404 case is 0.540" dia at the case head, and the ultramag case is 0.550" dia at the casehead. So, the ultramag case is really too large in dia to use in a 404 based round, unless the chamber is cut larger.
There is another wildcat series you might want to look at that is a std length based on the UM case, and in 416, 458 and 475, the AR.
http://www.weaponsmith.com/AR-rounds1.html