I don't think it's as difficult as you think. The GT350R actually handles better than the ZL1. The base ZL1 was only 0.39 seconds faster around Big Willow than the GT350R, and that's not even a technical track. The ZL1 1LE with all that extra power and suspension tech was "only" 1.81 seconds faster around Laguna Seca than the GT350R. Ford could simply add power and some minor suspension tweaks to the R and have the ZL1 1LE covered. I think they might actually be aiming for the GT3 RS this time. And we all know that car is an absolute monster. Now that is no mean feat, especially considering how porky the GT500 is being rumored to be.
755HP/715TRQ < 772HP/743TRQ
I hear you, but the GT350R doesn't have the added nose weight of a SC, 11 intercoolers, all their plumbing, and 650HP worth of engine cooling. so the GT500 will have the same challenge as the ZL1 1LE: continuous duty cooling for even more HP, weight-distribution management and total weight (Mustang is starting from a base that I think is about 100+ pounds heavier than Camaro). As a result neither the 1LE nor GT500 likely can match the GT350's handling dynamics. Still 750+ HP can 'forgive' some sins but only if Ford is clinical in managing weight distribution as effectively as the ZL1 1LE did and I expect Ford will, yet the extra base weight and the +100HP will add even more challenge to the nose weight (radiator and intercooler capacities) ...no simple task.