Ok, will it is true, the Drew's are tall. Still, you only get one chance to have the full intended height. If you cut the legs, you are stuck with that choice. I find we use the seats in two ways. First is slid all the way back with a boat cushion against the bulkhead as a seat back (if you have a square certified throw cushion and tick one corner between the back of the seat and bulkhead it will stay there). Second, when fishing you tend to perch on the edge which is stable and comfortable.
In the end though, and with the 16, the limiting factor will be available surface for the mounting plates. One of the best qualities about these seats if they can be mover around and removed all together We have installed brackets in the aft corners where the seat can be sat upon sideways. On nice days this tends to be the most popular seat running or adrift. The fiberglass lip above the gas tank valance is correct in terms of the full height. A brace at the base between the valance and the tank creates solid support for the base of the seat's legs.
Anyway, the first thing I would do is identify every surface where I might want the seat, note the mounting limitations and then select a seat height that gives you the most flexibility. With two seats, I toyed with the idea of two separate heights, but chose to keep them all the same.