We'd all love to help, but need more information (see Ron's post) - what year is the car...it makes a differnce as the bushings on the rear axle change in '67.

If you think about it, what you've described is a case where something is moving under load. When you release the clutch and apply torque to the drive train, something is moving. In order to get a clunk, things must be relaxed at rest and then when torque is applied, they shift as a result of the application of the force. Look at a loaded 18 wheeler moving from a stop light and note how the entire frame twists under torque.

So, back to the problem. What could move? Clearly the engine and transmission is going to be twisting against the applied force. They are connected to the car via 3 rubber bushings (two on the engine up at the front of the cross member) and one on the transmission at the rear). Inspect them for wear, if they look like they are still inline that is a good sign. If they look like a stack of quarters that someone has pushed over...that is not a good sign.

Try to wedge the engine up...if it moves, the mounts are shot. Replace them. If not, then keep moving back. Next up is the two (again year of car would help) small rubber bushings that connect the front drive shaft exiting the transmission to the rear shaft. If you look up, you'll notice that there are two pins attached to a plate that rotates on the end of the front shaft (just behind the big nut). If the pins are sitting in good rubber bushings, then that can't be what is making the clunk.

We'll have to assume that you've had the slip joint (spline) checked when you changed the u-joints. Next up is things connected to the differential/rear axle. Start as I suggested, with the bushings. First up would be the torque rods. They are out by the wheels and connect/locate the axle to the car. Grab the rod (again, year matters here as the '67 is different) and give it a good shake. Any noise? Inspect the front and rear (removal is not that hard and well worth it in this case). Then check the locating link same procedure - do both sides.

The panhard rod (goes diagonally from axle to body) can also be checked by the grab it and give it a good shake method. Nothing should make an audible noise or produce much movement (best done as well with weight on the wheels, not suspension in the air).

That should get the bushings checked.

Craig