So, to all who are interested in doing this little project. TXKid said the 5/8ths are all you need, so perhaps it varies a little in the 4th gens based on year, because I put in a 3/4 in spacer, and it was not enough. I think someone above stated they put in a 1 in, and it worked. Based on my examination of the hooks, I think a 1 in spacer would be adequate, however even with just a 3/4 in spacer under mine right now, the bolts are at the limit, i.e. if I was going to put in a taller spacer I would need new seat bolts that were at least 1/4 in longer. Also, the head rest gently brushes the roof of my truck now that the seat is 3/4 in higher, and I suspect a 1 in rise would cause it to brush, not so gently.
There are a couple bonuses to doing this though. First off, 3/4 in makes a BIG difference in what you can store under the back seat in the little space behind the fold flat floor while still allowing the seat cushion to come down without having to force it. Secondly, if you have thick-ish floor mats like the stock rubber ones your fold flat floor will now be flat, other than in the middle where it will now be low. One more spacer should fix that, and I will be much happier since it was always leaned back up to this point. Another thing is that it slightly changes the angle of the seat cushions relative to one another making the back seat more comfortable, plus sitting higher gives you a little more leg room (vertical shift), which in my quad cab is an valuable asset.
Here is what I did with mine though. I put in a 3/4 in spacer, and then trimmed about 1/4 in off the hooks on the back of the seat. I used an angle grinder with a cutting wheel, but there isn't a lot of material there, so you could probably just use a set of bolt cutters. Once this was done the seat indeed folds forward, and the hooks lock it upright when the cushion is fully down, or fully up. However, when folding the cushion up, or down the seat back does try to fall forward, so if you want a "hidden" storage space you will need to use bungee cords or something to restrain the seat-back during folding. If you don't really care, putting your hand against it while folding will more than suffice.
The spacer I used was a 3/4 in thick piece of particle board with acrylic on both sides. I cut four strips 1.5 in x 13 in, and drilled three holes in each down the center line (3/4 in from both sides). The holes were 3/4 in from the end, 2 in from the end, and 12 in from the end. The 2 in dimension is slightly estimated, so I recommend re-measuring the placement of the anchoring pin to determine the exact location of the third hole. Then, bolt it in, and trim the back hooks.
Hope this is helpful info.