Nice - I never tried this with Blender or other Z-up apps so didn’t think to add the axis rotation for it to GlueP, the conversion tab in the shader is meant for converting camera-space to world-space really and I’m not sure it will do exactly the right thing 
Looks like you figured it out but if in doubt you can rotate the pixel values using Ls_Vops instead:
I also just took a look at the FOV difference because it should line up exactly really - I think there’s something weird with the way Blender exports cameras to Alembic because if you look at the Film Back tab in Flame on the imported camera, the aspect is 1.5 where you’d expect 1.77777, and that affects the FOV. If you use this calculator: Camera Field of View Calculator (FoV) …to get an FOV number for a 50mm lens on a 36x24mm back set to 16:9 crop the number you get is 22.9 degrees, and if you use that in GlueP it does match exactly. Kinda annoying, haven’t had that issue with Houdini/Maya…
Glad it worked eventually anyway - it’s super handy but there are soooo many things to go wrong setting it up, I usually end up opening the 3D scenes and exporting the camera/rendering the P or Pref passes myself to make sure they’re right 