XJS Trans Mount

Howdy, I have an 86 XJS that I just replaced the trans mount on. I used a urethane bushing, replaced the spring seat but used the old spring. I used calipers and checked the spring length against the new one I purchased. Exactly the same, so I used the old one and saved the new one for "next time". Now despite the new bushing, I still have a "thack" under certain road bump conditions, though very much improved compared to the "foam rubber" bushing pieces. Does this mount need to be adjusted in some way? Perhaps I did something incorrectly. I soaped and fitted the new bushing to the mount plate. Used only one spring cup on the bottom. I saw that the mount plate had eccentric holes that had about 1/4" "scoot room", perhaps moving it around slightly. Anybody have any sage wisdom for me please? Adysthemic