Unfortunately, most aftermarket seats are thicker than stock seats.
A slider alone might be enough, because it slides back and down.
I've got a half cab on mine (for now), and the high backed seats ran in to the window, so I could only slide back part way. I went with Bestop low backed seats, and they slide back a lot further, and I'm pretty comfortable (I'm 6'4" as well).
The seat padding is still pretty thick though... I'm hoping it will settle over time.
There is also a thread in the tech section that describes how to lower your seat brackets, if you don't mind cutting and welding.