Yes the '55 motormount with the one bolt should mount to the '57 crossmember. You would only need the timing cover mount plate if you were putting it in an earlier GMC. I'm assuming the V8 and 6 used the same crossmember as I don't have a V8 one to look at.
The 270 came from the factory set up for a full flow filter. At
oldgmctrucks.com you'll find a plumbing diagram as well as a wealth of info on the engine including info on the various draft tube/pcv set-ups.
