I'd have to vote 408 here too considering this is going in a heavy truck, the only way a 347 would be comparable is if it gets a blower. If you want to keep EFI you'll be replacing or upgrading everything EFI and fuel system related from the fuel pumps to the injectors and fuel rails the intake manifold and the computer and portions of the wiring harness. But with that done and a tuner module attached to the computer you can dial it in from the drivers seat on a laptop.