[455-4+1]

The measurements are to a datum line, not the floor or a ride height dimension.
I thought that at first as well as 7/8" seemed an odd dimension. At first I held the b and c dimensions, but could not get anything to line up
From the pics you can see that d, e, f, g are a within 1/16" measured to a laser datum line and agree perfectly, so I determined that they were correct.
Yes, the difference between b and c are the combination of the firewall bushing and the frame ear thickness, which makes sense, it's just out by 3" or more??
If someone has a car stripped at present I would really appreciate a check measurement.
A quick one would be to use a straight edge between points d and e (use blocks cut to these dimensions to space the straight edge) and extend it past the firewall mount. Then measure the height to the mount pad.