Eliminating rear ledge?

I am building my pizza oven on an existing kitchen that has a depth of 45”. My question is if I can adjust the size of the 56x47” cooking base slab? How much extra room is given on the rear of the slab? Is it 8”?

Could I shorten the slab 8” or whatever the exact measurement is to leave 0 ledge in the rear? If so, where would I start the rear of the form? The original design says start 15.5” from the rear. If I eliminate the rear ledge where would I start my new form, 7.5” from the new rear?

Also can I replace the front brick ledge with a 2” bluestone tread? I need it to over hang the concrete slab so I can veneer over the concrete slab.

Hope my question was clear, thanks in advance.