It is the mount. I had the same problem on my 67. Here is what you can do. With the car in an open driveway have someone or do like I did. I used a camcorder on a tripod to watch the engine. Don't stand in front to watch it!!! Now apply your brake hard, put in drive and gently apply gas. If motor mount is broke your engine will rise on the drives side. Mine went up 4+ inchs, this causes your gas linkage to open up the throttle more!(cool video) BE READY TO TURN THE IGNITION OFF!! It may want to runn away. If this sounds too scary then just use a block of wood on a floor jack under your pan (do not try and jack the whole car up, u will crush the pan)and if the engine raises more than a 1/4 inch replace the mounts.