I actually stole parts from a similar Grand Cherokee (and Magnum truck) at PickNPull for my 65 Dart. I used the underhood relay box, mini-starter, battery cables, and distributor (down-the-road plans).
Back to your question, yes, just run a bypass wire around the ammeter in the engine bay. No need to pull your dash cluster again. An 8 awg wire won't show much on the ammeter. A 12 awg wire might show ~30% of alternator output current on the ammeter (depending on length and connectors), which might be a good fit if you install a 100 A alternator. I wouldn't go smaller than 12 awg since I recall that is what is wired to the ammeter. Even better is a "variable resistance wire", which is what the parallel diodes I used realize. It shunts a higher percentage around the ammeter as current increases.