Brian; not 100% sure but it looks like you have the valve plate with the 9 holes in it placed to the outside of the valve body. It should go between the valve body and the cylinder body. Then the cover is the same size but without the 9 holes.
Pretty cool to see all the parts modeled and exploded.
There is a web site I came across quite a while ago that has some great information on steam engine design, including many "Rules of Thumb' for calculating valve sizes, piston rod diameters, etc. It might be of some interest
In both cases, just follow the links to the simulator programs. They have to be downloaded and installed, but they are both "safe", many people have used them.
They cover both inside (usually piston valve), and outside (usually slide valve) gears, and if nothing else, will repay your time "playing" with them. That said, they are a bit deep, and may be beyond interest unless you are keenly wishing to learn about valve gears. Have a look, and make up your own mind.
I know squat about steam engines :big: but I ran across this site in my daily internet travels and was reminded of your question about valve timing - it may or may not help;