David, beyond the grill's ability to support the weight of the bricks/cement, I think you need to take another look at the Frankenwebber thermodynamics. His setup has gone through a few different iterations, but I believe that he's always had the fire underneath the hearth rather than on it. There's no space for a fire on a hearth of that size. With a 26" grill, it looks like you have about a 20" x 24" hearth. There's no way you're fitting a fire and a pizza on that.
Not that a Frankenwebber is even worth aspiring to, imo. If he was more forthcoming about his materials/setup, there'd be hope, but, right now, everything Frankenwebber related is mostly conjecture. LBE's, on the other hand, are heavily documented. A 26" grill could give you a lot of real estate for an LBE.
Are you 100% certain that he's working with portland with direct flame? Everything I've read on portland says not to do this.