Jake, you don't have to pull the engine to stab in a new cam, but you will have to remove the radiator and grill to get room...maybe the hood too. You can run the engine stock as is, no worries there, but when you finally do change out the cam, you'll have to install new lifters. That is more involved and requires removal of the intake. It's more work doing that way, but you have to choose.
I wonder if changing out the cam now, when the engine is new, will void the warranty? My guess is yes. Buy a GM engine that fits your needs as close as possible so you'll have nothing to do for a looong time. My $.02
Davy