Where to paint vs undercoat per factory?

[BuzzNectar]

Barracuda is going on the rotisserie in the next week or two 
My body guy was wanting to find out per factory spec, where it gets under coated and where it should be paint underneath. Even though the majority of this resto will be very close to factory except a few things like keeping the headers, aluminum intake (still have the original) and having a power bulge hood (Gran Coupe) I wondered if you guys think undercoating the entire underneath would be a better decision? It is a numbers matching N code 383 Gran coupe, In Violet with lots of factory options. Upon completion (hopefully this June) we plan to go to cruises and shows the remainder of this season, then put it up for sale.