OS Valve Timing
Here is a tip you may want to try if your uncertain the cam timing mark is in the right spot? Don't quote me on numbers here. The .40 FS for example has 22 teeth on the cam gear, the crank has 11 for a 2:1 ratio. 360 degree's divided by 22 = 16.36 degree's for each tooth on the cam. The way I look at it, there is three ways of putting the cam in. One is right, two are wrong.
Install the cam, (you have to rotate things slightly because of the helix of the gear teeth) if it's right, the mark will be slightly angled back and almost parallel to the axis of the push rods as previously suggested. Now if you remove the cam and advance it 1 tooth or 16.36 degrees, install it, you will see that it is totally out to lunch. Remove it and retard it 2 teeth (1 tooth to the other side of the correct position) and it will be totally out to lunch again. Remove it, advance it 1 tooth, install, and your back in business.
Another suggestion, before you tear the engine apart, make a primitive degree wheel. Time it with TDC on the compression stroke & the mark on engine case. Rotate the crank through it's valve cycles placing marks for the opening & closing of each valve.
Now after you do your thing with the engine and reassemble it, you have a reference to check your valve timing. If your out 1 tooth with the valve timing you should easily notice it. Before you start your engine that is. Good luck with rebuilds.