Where to look
Pictures and measurements based off of driver side transmission cross member rivets and holes
Most all trucks seem to have a combination of 6" spaced pair of holes and 5 5/8" spaced pair of holes. Some trucks have these pairs of holes spaced differently relative to each other.
Below shows these pairs of holes and you can see that this particular truck only used the 5 5/8" spaced holes. It does not matter which set your truck uses.
1. The distance from first hole to second hole will be 1.8" +/- .1".
2. The distance from the rear flange of the front cab mount to the front of the 6" spaced holes will be 11 5/8" +/- .1".
3. The distance from the bottom of the frame to the center of the rear of the 6" spaced holes will be 2.25" +/- .1". there seems to be a decent amount of variance in this, that's nothing to get worried about- except if you have a supercab double check that you dont have "x25" shown below