Mount TV in stone facade or drill through to chimney block

I'm mounting a television above my fireplace mantle. I know about the heat and neck strain issues, and they are outside the scope of this question. I am looking for an answer as to whether the stone facade will support the TV, or if I should drill through it to anchor the mount into the block of the chimney.

The stones in the facade are relatively thick, averaging 6" (15cm). They are laid and mortared in front of 1/2" (1.2cm) plywood sheets. The sheets are in front of the actual block of the chimney.

If I were to go, say, 3-4" (8-10cm) into the mortar and use concrete anchors, would that hold it? Or is this a better-safe-than-sorry situation, where I need to get some all thread rod (ATR) and go through the mortar and plywood, anchoring into the chimney blocks? If so, could anyone recommend anchors to put on the end of the ATR? Most of the anchors I see have the bolts already with them, and I'm not seeing any online that are 12"+ (30cm+) long.

Here are some pictures–first is the front of the facade, for reference. Stone is 8' (2.4m) wide, by maybe 12-14' (3.7-4.3m) tall. Note the power wire expertly run through the mortar just left of center.:

Looking in from the left, behind the facade. Note that same power cord coming through, for reference. The plywood sheet on the right of this picture is behind the stone facade:

Here is a profile picture of the facade, with tape measure for scale. Here you can see the stone facade from the side.:

I put up an album of full-resolution pics here: http://imgur.com/a/qpDue

If it's relevant, the house (and presumably the chimney) were constructed in 1976, 40 years ago.

Long time reader, first time poster. Please be gentle. 🙂

EDIT: The TV weighs 50 pounds with the stand. So I think 35-40 pounds is a fair estimate for the TV alone.