Clearance from top of stove to underside of masonry opening

Clearance to the brick? I think the stove could touch it. Your mantel, if combustible, is another story.

My F3 is about half way out. The clearance to the lintel is about 2 inches and have no issues. I have a heat shield to protect the underside of the wood mantle.

Time to change the avatar picture maybe?

I think you'd be ok. The surround of most inserts touch brick and some get plenty hot. Not an issue, matter of fact off hand I'd say it just adds to the thermal mass. Can you post a pic just to be sure? Then everyone can get a look.

One thing. Check that the brick is mortared all along the lintel. Mine was not originally and heat and smoke were able to get behind the brick and came out under the mantle. This was back in open fireplace days.

My F3cb sits on the mantle , the lintel is 3/4 of a inch above the stove pipe . I don't think there is a minimum for the stove but I do recall a minimum for the stove top to a combustible mantle ( 3'-0" ? ) . We had to remove the wooden mantle to meet this criteria . I included a photo so you can see how it sits we did have to purchase a hearth extender ( 18"x48")to meet the code . So far we have really enjiyed the stove and the warmth it provides .