The terrain following problem will be caused by the lower lift arms. Even set to float, they're not likely going to rise and fall in time with the speed you'd ordinarily work a harrow. Unless your property is flat as a pool table, I'm not sure 16-18" retraction on a HTL will be effective either. That's not a whole lot of slack in a pair of chains that will likely be at least 9 feet long on the diagonal. There's a LOT of stress too. I normally use mine with the tines in agressive stance, and regularly bend lift pin brackets and light duty toplinks. Keep in mind that expensive HTLs are most vulnerable when fully extended.
If you follow your intended design, AND you engineer strong enough lift pin mounts, AND they're far enough away from the rear wheels - the lift arms will raise the FRONT of the frame without hitting the tires. But the geometry you describe, still suggests that a HTL will not pick up the back end far enough to keep it from dragging - IF it picks up at all. And it may also lift the back end when you don't want it to - like when the tractor crests a hill.
With the expensive factory TPH frame versions, you sacrifice some terrain following ability for lift. With the much less expensive draw bar versions, you get aggressive terrain following - but NO lift capability. My version ended up as a reasonably priced compromise. Terrain following is only limited by the amount of slack in the lift chains, and lift capability is just as effective as a factory model. A little ungainly looking perhaps, but effective. I can also spin the frame 180 degrees to point the teeth for a less aggressive bite, or turn the whole thing upside down - for no teeth at all. Don't think the factory jobs can do THAT.
//greg//