Grand piano is one of the more demanding instruments to record if one is critical about their sound. There are a million ways to mic it depending upon desired sound, microphone types, quality of room, etc.
That Samson could be tough to make sound good with the grand, as would the 58s. If you have a decent room, you might want to first try using just the AT, setting it up outside the nearfield of the piano (i.e. farther from the piano than the length of the piano itself) to get a good, full sound of the whole instrument. The lid should be wide open for that appliction, with the mic facing in towards the underside of the lid.
If that is not a good option, try micing the AT inside the lid a few inches above middle C, 8"-12" below the hammers. Have the AT angled down to a point about halfway between the mic position and the hammers. Adjust this position left or right to emphasize the high or low strings, and forward or back a few inches to adjust between the honkey tonk hammers and the mellow strings. Adjust lid halway or full open to desired sound.
A third position would be at the heel of the piano, lid open, just outside of the piano boundary with the mic facing in towards the piano innards and the player. Here a few inches in any direction could make all the difference in the world in sound.
If you have access to a place that rents microphones, you might be best off getting a matched pair of condensors (large or small), using the second instde the piano option. Set them up as an X/Y coincident pair to get the full rangs of strings in a stereo spread. You can pan to taste, narrow or wide, but you'll get a much wider frequency spectrum than you would with a single mic. As long as you're renting, get a third LDC to place outside the nearfield as described in the fist example and use that to help fill the sound.
HTH,
G.