Not sure if it was related, but years ago, I had a 390 in my 64 Falcon, mounted using 2 "ear" style aluminum motor plates using the water pump bolt holes, with studs. I also used a factory rubber transmission mount, and no mid mount. After a while, the C6s bellhousing cracked, as did the flexplate. A chassis guy I know, suggested that the large span between the front of the engine block, to the trans. mount allowed the engine/transmission assembly to sag enough to cause the cracking, and he recommended installing a midplate to give additional support. I never did, as I sold the Falcon as a rolling chassis shortly afterwards. I can say the aluminum plates, with a gasket on either side, with RTV, never had any leak issues.