Here is a simple tool that can be used to check the timing marks on your XJ6. After pulling an engine out of a wreck and finding the timing marks 60 degrees out, I thought I had better build a gauge that could accurately determine how the timing marks on the crank damper related to the piston position.
The engine that was 60 degrees out is an extreme case, but with the elongated holes of the timing mark plate, it is possible for this to set up to 4 degrees out, even if the damper is properly aligned.
The gauge is very simple, an old spark plug with the electrode and ceramic removed + a length of 5/16" rod. My gauge uses a spark plug I adapted as a leak down test unit so also has a fitting to attach an air hose. The rod should be nice and straight so it slides easily in the plug body.
The gauge makes use of the fact that when cylinders 1&6 are at TDC, then 2,3,4 & 5 are all exactly at the same distance below the top of the deck. Just set the crank at TDC and fit the rod to say No2, make a mark on the rod at the top of the plug body, then do the same for No3. I found my road car was exactly spot on, which was reassuring.
I believe this method should be accurate to around 1 degree.
The first photo shows my gauge sitting on top of the cam covers, the next shows my "spare" engine and how far out the marks were.

 

Good work John!