The X block was the over the counter race block that obsoletes the former TA block. It had thicker decks, partially filled pan rail, some came with 4 bolt caps, some with 2. Could be either straight or angled outer bolts. It came with a bore that measures about 3.870 or so, with a rough bore so you could finish it at 3.910, but I've sonic checked a few and most would take an over bore of 4.125ish.
It also had 340 mains, takes OE cam bearings and has OE tappet bore angles.
It's a nice piece. Its biggest PITA is it has the 318 drivers side motor mount, which is not the same as the 340-360 drivers side mount. Passenger side mount is the same.