Chugging/vibration/noise at highway speed

1974 Dodge Dart Swinger. Newly rebuilt 360 bored .030" turning a bone stock 904 and 7 1/4 stock rear end.

So, you can read about my rebuild of the 360 in the forums. It went well, we've been driving it a lot and I really like the power it's putting out, but I think it's leaving something on the table. It seems to have a lot a torque and gets up to highway speed pretty quickly but once you're there it feels like it's chugging like a freight train. It's very noisy and after an hour or so, I have a headache. You can feel it in the steering wheel, floors and seats, everywhere. It's not a pleasure to take on the highway.

I've had a wheel alignment and balanced all the wheels too.

What I haven't done is touched the transmission, drive shaft or rear end. Could one of these be the cause of the chugging/vibration/noise? It just seems like there is something wrong, but I have no idea where to start beyond the engine. How does one determine what is causing it.

I've checked the timing and it's bang on, plugs are nice and gray/brown after 1000 miles.