273 adjustable rockers on 340?

You can run solid flat tappets on a hydraulic cam. You cannot run hydraulic lifters on a solid flat tappet cam. There is a little power to be gained by doing this, but you're more apt to get it from the lighter lifter, and no hydraulic dampening affect rather than any specific gain in lift or duration.
My buddy used to trap his Ford at 8000 rpm with a hydrailic cam. He ran 130psi oil pressure to essentially lock the lifters at zero preload and custom pushrods (in a Ford).