Not sure why the bow seems to like 50/55's if it prefers weaker spines, But it doesn't really matter:I do find my broadheads sometimes make the arrow show weaker than field points. What length are the 50/55's you're trying?.
Anyway: For 27.5 BOP, you'd cut 'em at 28.5. To be on the safe side, I'd cut one at 29" first, and tune from there.
A set of shafts right around 300 grains, full length, should end up with a 28.5 BOP finished arrow very, very close to 425 grains finished.
I'd get premiums in whatever you choose, to get a close spine and weight matched set. Snag is a great source here, as you can read by his posts. I didn't know he was still selling shafts. Raptor also carries spruce. Must be others, but those two come to mind. Also, keep in mind that if you have a set of shafts tapered,it'll bring the weight down by around 20-25 grains per shaft (and lower the spine a pound or two).