How much space above the fire to the register plate?

I have a chimney breast that has been knocked out and a steel RSJ inserted to hold up. Acutally, I discovered the RSJ when I knocked out the rest of the breast/fireplace. It sits just below the original soldier-arch.
I intend to place a small wood-burner under and take the flue from the stove through a register plate etc. The usual format.
What I need some thoughts on is where to put the register plate. I can put it below the RSJ (resting on the top of a decorative oak lintel) and then back-fill through the small soot box I have installed in the face of the breast.
Or, I can put the register plate much higher up (akin to inside) in the brick flue/breast. This will significantly reduce the amount of back-fill (if any) but will mean that there will be a considerable void above the stove, before the register plate. My main concern with doing this is that the void could get a heat build up, which would then warm the RSJ too much, causing unacceptable expansion. There will still be the decorative oak lintel below the RSJ though.
Does anyone have any experience with this sort of scenario? I have done a few similar arrangements before, but never where an RSJ lintel has been present. Normally, I have made use of the soldier arch or made the oak lintel load bearing.
Thanks for any pointers/thoughts. Rob
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Kalico wrote:

Yes. Almost exactly the same, except it was a new build, and teh soddng builder biutl the steel backed oak beam lintel so low we could barely get the stove IN and it HAS to have th register plate up much higher han the lintel.
In practice, the hot air collects at the top, yes, but spills out very quickly. There is no problem with the steel expanding over such a short run anyway.
I'd say do it to your taste and don't worry.
I have done a

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