I finally figured out what the problem was! I originally thought the blades were flexing and rubbing at high speed, but when I took it apart there were no marks on the housing. Well it turns out I was on the right track, but I was looking in the wrong place. The blades of the impeller are riveted onto the disc, and it's those rivets (on the back side) that were rubbing. I thought I had plenty of clearance all the way around, but apparently there's at least 1/4" of flexing going on at full speed. So I took it all back apart, routed out 1/8" or so from the spacer to get more clearance, and put it all back together. Works great
Now for the blast gates...