Don't overlook your motor mounts, especially the rubber biscuit mounts that came on the '67's. Here's the easy way to test them. Open your hood and start your engine, you should be able to see the engine or air cleaner through the gap between the raised hood and bottom of the windshield. Now put the trans in low gear and power brake the engine for just a few seconds. If your left side motor mount is bad you will see the left side of the engine raise up a good bit from the torque. Now do the same thing in reverse and watch the right side. I've seen them so bad the engine torques until the exhaust bangs off the floors. The rubber insulators will appear good until you put the torque to them, give it a try.