[unclesalty]

something I came across on the internet (so it must be true) sbc had metric starter bolts on the later blocks (gen IIs?) they were slightly different diameter wise, if you mixed them up, the extra side play from the smaller diameter bolts will allow the starter motor to cock. I guess there are adaptor sleeves you can use to correct this.
(The starter bolt shoulders are knurled so they fit the bolt holes snug)-just something else to check, wont cost you anything- you may have gotten an oddball part (re-man) somewhere, it happens. Good luck. Usually a kickback= too much timing, too far advanced.Check you plug wires unless she runs real smooth-