I went over to Paul's house today. With his guidance I was able to install the valves, springs, collars, and keepers into the heads. Before doing that, though, he showed me how to lap the valves on the seats to ensure a good seal. It's a very easy process, though a bit time consuming. Unfortunately I didn't have my camera with me, so I had to make do with my phone's camera. They didn't turn out too badly though.
Here you can see an unlapped and a lapped valve (the dull part is the lapped surface):
Here you can see an unlapped seat and a lapped seat (the lighter color is the lapped area):
The process is simple: Attach a lapping tool to the face of the valve with the suction cup, apply the lapping compound, insert the valve, and spin the tool back and forth, like you're trying to start a fire with a stick and tinder.
Once that was done, we assembled the valves in the heads and then poured kerosene into the intake and exhaust ports to ensure that the valves were sealing properly against their seats. We found no leaking!
Next will be the rocker box covers; I'll hopefully get to them within the next few weeks.