GunslingerFIN, The thicker mount is correct for the 64-66 a-body if you use the thinner mount, it can change the driveline angel depending on other modifications performed to install drivetrain and can cause header contact to frame. Now it's time to look at other areas, trans mount looks different between 64-66 to 67-72 I'm not sure on height difference, it could be causing close header to frame contact and your picture looks like it's closer to middle of the trans area where the tube run under the frame. Is the trans crossmember rear suspension in stock locations? It's time to look at other items and let us know what you find.
