The Pantera guys in the US run engines that robust, and their ZFs hold up. Maybe a ZF that is built properly is the way to go- it would fit from the beginning, unlike the others, and the linkage is already set up. You might talk to Dennis Quella at Pantera Performance in Colorado, I think his Pantera has that much power at least.
800 HP is pushing it a bit, but the ZF will withstand it IF it isn't abused. By that, I mean that it isn't speed-shifted, or you don't sidestep off the clutch in first gear, etc.
The Byars Brothers have been putting stroker 460s into Panteras, which is a big job as it moves the gearbox back far enough to require CV joints, because U-joint driveshaft angles are too acute. Engines as much as 572 inches (I believe) have been used, well up in the 700-800 hp range.
The guinea pig car for this conversion was seriously abused. Don Byars used it to show how much faster a Pantera could be, vice 'only' a 550 hp stroker Windsor or Cleveland. He would do burnouts and full-throttle launches from stoplights, again and again. Eventually the ZF cried "enough" and dismantled itself in the street. But Don freely admitted that he caused the failure, rather than attributing it to some defect or inadequacy of the gearbox itself.
Dennis Quella has a tube-framed Pantera replica track car, and he runs it really hard. But he's respectful, and doesn't seem to have any gearbox problems.
Gregg Esakoff drives his 750 hp race Pantera like "Animal" from The Muppet Show plays drums. He's REALLY fast and REALLY abusive, and he manages to break his gearbox in ways that nobody has ever seen before. He never uses the clutch, so it's no surprise when the thing only lasts a half-dozen events or so.
But the bottom line is, whether in a Pantera or a GT40, as long as it's subjected to no more abuse than the Ford teams dished out racing at Le Mans, they should be fine. And the reality is, if you're just driving on the street and/or the track, and not deliberately doing something stupid, the ZF will almost assuredly outlast YOU! :laugh: