W760 being a ball powder I would work up the load with magnum primers, as I have almost always had a little better consistency using them with ball. I have not had a round fail using standard primers/ball either so don't loose sleep over it. The important thing is to stick with using what you worked up the load with, or back off a little and work back up if you substitute afterward.
I use H414 (same powder IIRC) in the .243 with 75gr bullets but haven't loaded any 55gr, sorry.