I was finally able to track down a current Oregon catalog online:
Oregon Chain 2010 Catalog
Based on this, I was able to narrow down my options and finally settle on the 96-354 blades. The 90-348's are also listed there are a direct replacement for the M145476 blades... they do not have fins, are are not high-lift blades.
The other likely option, the 96-377's, have a center hole that's an extra 1/32" larger, and the blades are also 1/32" longer. I didn't want to worry about the extra length and slop causing the blade tips to hit, so I scratched those.
Ordered the 96-354's from mowpart.com at 4:45 PM on Wednesday, and they were delivered by 2:00 PM on Thursday with standard ground shipping! I was amazed...
The only thing I'll say, is with the coating that's on them, they didn't seem anywhere near sharp enough to me. I ran each edge across the bench grinder once, and they were knife-edge sharp after that.
Put them on the x324 last night (48C Deck), with the Deere mulch kit still on, and they fit perfectly. No rubbing, blade tips clear, and still overlap enough side to side.
The grass was still pretty damp, but I decided to give them a quick test anyway. Cut quality was definitely better. They didn't really seem to 'lift' as much as I'd hoped where the tires had matted the grass down, but considering how wet it was I'll give them a pass on that, and see how they work when the grass is dry.
Mulching quality was fantastic though, especially considering the wet grass. I didn't see squat for clippings on top, and the mulch kit kept everything from blowing all over my patio and flower beds.
A lot of people had commented on the noise... these may have been a little louder than the previous blades, but not enough that I would have thought anything about it, if others hadn't made comments.
Thanks for all the advice.