The late spindles, all those other than the 83-85 944 NA, have a wider bolt spacing by a few millimeters and the knuckle (the part of the spindle that fits inside the strut) is a few millimeters thinner. It's the 83-85 944 spindle that will bolt into the 1303 suspension, the use of the later spindle would require the modification of the strut and/or spindle to accommodate those aforementioned few millimeter differences.

The use of any 924/944/951/968 A-arm and ball joint would require a major modification and alteration of the Superbug's suspension and chassis, it is far cheaper and much simpler to just use the Kerscher ball joint on the 1303 control arm.

EDIT: forgot to mention that the 83-85 spindles have the hole for the speedometer cable and the 1303 cable will fit, the late spindles do not have this hole because the later Porsche's speedometer cable was hooked to the transmission.