I agree with 3D printing or the other method. Having to use actual N64 cartridges is silly since these won't be functioning N64 cartridges anymore anyway. It's a grey piece of plastic, just make new ones.
I agree, Injection Molding sounds nice. It won't be a working cartridge anyway because it has to be gutted.
I understand the authenticity argument, kindof... but I think something completely original that can only be received from this kickstarter would hold a lot more value to a lot of people as a collectors item. It'll look enough like an N64 cartridge, but it'll be original, you know?