Hey Bruce,
Let me see if I have this right. With the plugs out, and the valve train properly lashed, you can rotate the engine over no problem right? But when you install one plug, any plug, it rotates and then stops when the cylinder with the plug installed comes up to tdc. Are you rotating the engine with the starter or with a breaker-bar and socket? With only one plug installed and a healthy bicep you should be able to continue to rotate the crank past tdc with a breaker bar and socket.
This may seem like a stupid remark, but you do have the correct length plugs in there like Champion N10Y's. A longer plug could be acting like a piston stop. No wait, you said you can turn it a little more if you let the compression bleed down, so never mind about the plug length thought!
You tried a different starter and that didn't help so it may not be the starter. Battery up to the task? When you crank it over with the starter does it seem robust or weak? Did you check for proper voltage at the starter? I forget where you have your battery, but are you sure that the cable gauge size is correct for the length of the cable run?