I had the same "mod" done to my jeep when I bought it. I ended up finding some used factory seat frames and refinishing them. At most of the sway meets I went to, the factory frames were going for $75-100 each depending on the condition. For the coverings, I went with Beachwood Canvas and went with 2A cushins instead of 3A. I'm 6'4" and the 2A cushins are about 3" shorter so they give you a "little" more legroom. Like Carl said, you still better wear knee pads if you take a long trip

If you are not wanting to look stock, you can take a later CJ seat frame and bolt it on top of your factory seat bottoms. This was the mod that the previous owner was "trying" to accomplish with mine. If done correctly, it would scoot the seat back some and give you a shade more legroom and give you a taller, fullsize seat back. Mine was done very poorly, but the idea was sound.
If you would like pics, let me know and I'll send you a link.
Good luck!