2-3 shift slipping

So my 70 Duster with a 360/727 B&M Ratchet floor shifter has a slip in the 2-3 shift when I manually run the gears and shift around 3500 RPMs. It winds up and as soon as I let off the go pedal it locks in and holds fine. The 1-2 hooks fine even at higher RPM shifts. I had a friend help adjust the bands and it didn't make any improvement. When I put it in D and let it shift by itself it seems fine. Any ideas? I've got a spare 727 that is hopefully going to be built this winter and installed at some point.