answered some of your questions over here

http://www.4x4wire.com/forums/showflat.php?Cat=0&Number=1274444&an=0&page=0

I too did the valve seals in my 99 SOC. I did it with the heads still on the block, and all the main coolant lines still in place. Probably made it take a bit longer, but I didn't want to have to drain the system that I had just filled after a head gasket job.

Anyway, the most difficult part for me was removing the valve seal, followed by reinstalling the keepers.

The tool SCH91400B is exactly what I used. Again, rented from a board member for a fraction of the cost. Be sure to have a magnet handy to catch the keepers when they release.

Also, I don't think that pressurizing the cylinder head to keep the valves in place will work. Many of my keepers were stuck to the spring retainer or the valve itself. I had to apply some force on the valve via compression of the spring that would have certainly dropped it into the cylinder cavity. I ended up dropping 10 foot of flexible rope into the cylinder head (I did pressurize each cylinder to get it at bottom dead center), then turning the crank by hand to push the rope into the bottom of the valves. (google and youtube will give you a how-to)