Hi William;
I agree Matt did a fine job.
Yes, the bell-top temperatures with a Shorty Core stay well below 800°F, making the use of clay bricks possible.
Yes, using concrete blocks is fine at the lower level, and it cuts back on the overall cost.
In my opinion, bricks look better, but it's all in the eye of the beholder, and in their wallet!
A stone or tile facing would improve the look.
Ommm... Morgan Super wool is a non-ceramic fiber alternative.
Morgan also makes non-ceramic boards as well.
Shorty or any Batch core can sit on a brick plinth.
The metal frame is simple to construct and speeds installation, but there are other options.