Shorty Core & Bell under construction - some questions

hello all
I'm excited to be making contact with the inspiring people whose posts I've been reading for a long time

I am proceeding slowly with a build, having spent a lot of time circulating around the RMH world, reading, watching videos and learning.
It was worth the wait to evolve my aspirations from my first encounters with steel barrel RMH with horizontal flue pipe & thermal mass and from there to Walker cookstoves, but finally I'm thrilled to have found the Shorty core, got my hands on the SKP files, and have all the materials ready.

I'm attaching a few pictures of progress so far and hoping that some pair of experienced eyes will glance at this.

I have a 150mm flue to use as the chimney, and the core will be surrounded by a red clay brick and stone bell with a bench on one side. A bypass will be included in the bell.

For the box & riser I have dense firebricks - 230mm x 114mm x 76mm (9" x 4.5" x 3")

I've made the firebox 232mm wide (by using a complete brick as the floor) which is slightly more than the 216mm required but in all other aspects I plan to keep to the recommended dimensions:

Box:
width 232 / height 324 / depth ~430

Port:
width 54 / height 216 / depth 120

Lower riser:
150 x 150

Upper riser:
216 x 216

Question - does my wider firebox (232 instead of 216) cause a problem or require adjustment of the above?
I'm hoping it makes no difference but if necessary I can break down the existing couple of built layers and reduce the firebox width by cutting

Should I keep the riser exit opening at 216mm wide to match the riser or at 232 to match the box?

The firebox is built on an elevated base with AAC (autoclaved aerated concrete) thermally insulating "Aircrete" blocks immediately under, and below those standard soolid concrete blocks. The floor of the room is reinforced concrete.
The Aircrete blocks are suitable for exposure to max temp of about 500 Celcius. Is there a risk that temp could be exceeded on the external face of the firebricks under the firebox, sitting directly on top of the Aircrete blocks. There is no heat dissipation via air channels between firebricks and Aircrete blocks.

Hi Alan, your plan sounds solid, and it sure looks good so far!
I do recommend bringing your box width to the standard size.
Yes, on having the horizontal exit match the riser, even if you go with the larger-size box.

I'm guessing, but you might be ok with the aircrete blocks under the floor.
Perhaps use an inch of clay mortar between them.