A couple of different issues at hand here.
The “centering bolt” on older leaf springs use a smaller head/nut than what the centering bolt uses on FMJ springs. If I remember correctly, it is like 1/2” on older springs and 5/8” on FMJ springs – but don’t quote me on that measurement (just going by memory).
For a person to re-use FMJ leaf springs and older shock plates, a person will either need to enlarge the bolt hole on the older shock plates to match the FMJ centering bolt OR replace the centering bolt to older style springs. The centering bolt shaft diameter is the same, only the FMJ bolt and nut is larger diameter.
To change a centering bolt, a person needs to get two clamps and clamp down the spring (on or off the car) with differential removed. Tighten the clamps down snug (enough they will not “pop” off). Remove old bolts and replace with new bolts. Remove clamps and that job is completed.
Black - leaf spring
Red - centering bolt
Blue - spring clamps (should already be in place)
Green - one version of a clamp to use (C-clamp) but there are others that will work
Note: there usually is not any space between the leaves. I left the space for clarity.
It might be easier (and cheaper?) to just drill out the centering bolt hole on older shock plates.
There can be an issue of differential movement if you use a small centering bolt inside a larger hole, a person can’t/won’t be able to get differential in there straight and keep it that way.
Now the FMJ differential perch hole. The 8¾ (or Dana 60) spring perches are all drilled to the smaller/older centering bolt. In this case, if replacing both the differential and shock plates, I would use new centering bolts. No drilling would be needed.
If using an FMJ differential, a person will need to find a piece of metal and fill in the large hole (which I “think” is 1-7/16” in diameter – but don’t quote me on that). Once welded in place – re-drill the hole in the center. The side to side is not as big of an issue as the front to rear location is drilled at – but if using wider than normal tires, the side to side position might also be critical.
BudW