Is the high temp mixture required for the stucco layers as well as the brick?
Good question, Dave. The answer is emphatically “No.” Think of your oven build as having a hot (fire) zone, and a cold zone. The stucco is outside the hot zone envelope.
You can use inexpensive Type “S” (for “standard,” not “stucco”—fun fact!) mortar for your stucco. It sets up in a few days to a hard protective shell that will keep your mineral fiber insulation fluffed up and shielded from grandkids and heavy birds.
The base under the stucco on most builds is the standard-brick hearth frame (not yours, because you are constructing a monolithic poured slab). That frame does need the high temp mortar because it sits in a “twilight” zone that straddles the hot hearth and arch, and the outside cold environment.
Great, Thanks…I will be installing a double wrap of the ceramic blanket and I believe it calls for two layers of the Stucco/Type S mortar after that?
Dave
Spot-on, Dave. Remembering, of course, that after the blanket comes the metal lathe wrapping, a.k.a. chicken wire. I know you won’t be skipping that crucial step! 
Correct, thanks Matt.
Have a great weekend!
Dave
Is Portland cement okay to use for the stucco layer. I have 2 unopened bags.
Dave
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Portland cement by itself would be too hard and crack over time. I did a little research on mortars early on in my project that you can find here:
Since I posted that I’ve mixed all the mortar for my veneer using leftover Portland Cement and Lime. I needed to buy a lot more sand though.
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Ken is absolutely correct.
If you have left-over Portland cement and want to use it up, save it for the veneer as Ken did. He does point out that you’ll need other materials to make it a flexible and strong mix.
For the stucco layer I’d prefer the ready-mix as suggested upthread. The Lowes in Milford and Newington both claim they have Sakrete Type S in stock.