Personally I would just rewire the whole car IF the crimps / routing are not up to par (aka rats nest)
These LS swap harness kits from Painless appear the way to go, or a least a guide.
https://www.painlessperformance.com/wc/p.php?s=lscar
On the installation manual for this LS3 kit, Page 26 hows the starter / alternator wiring.
https://www.painlessperformance.com/wc/60524