That 1/2" pilot hole you are talking about is the "standard" reference and is used by Fatman's in their instructions as the hole to use for centerline for their IFS install.
When my straight axle was on, just from eyeballing it, that hole looked to me to be about half an inch off. Unfortunately, I pulled my axle before I pulled out a plumb-bob and actually measured for myself (yes, beer was involved).
I can try to remember to measure my frame tonight and see if I at least get the same numbers (it is a 59). As well, please refer to
http://www.classicperform.com/Instructions/53-56-Ford-Truck-1.htm which shows center-line to be 20 7/8 inches behind the front spring mount hole.
Hopefully some others will chime in on how they got to thier centerlines.
