The Yamaha 48 is pretty close to the Bach 6.5AL, perhaps just slightly smaller. I don't think there are many good choices specifically tageted at high range, since the Yamaha 48 and Bach 6.5AL are already on the smaller side for euphonium playing. I personally use a Wick 4AY (a 4AL with a small shank) on a 321. But the two mouthpieces you outline are actually pretty good choices. Focusing on making your embouchure more efficient by using your air as described by Rick is the best bet IMO.
Doug
Adams E3 0.60 Sterling bell - Prototype top sprung valves
Concord Band
Winchendon Winds
Townsend Military Band