exhaust pipe diameter for a mildly modified 340

That doesn't sound like a old pickup though. I remember watching those videos. That's like a Commando single exhaust. But if I remember right, that's not the "Hemi style muffler" and no resonator. Sounds great and he really get's after it at about 28 min. Downshift and go.
I believe it's actually a resonator and no muffler.. i'm too lazy to go look though :)