Try those new polyurethane engine mounts (Energy Suspension), they have interlocking flanges like the later model Chevys have, unlike the earlier ones which can be stripped apart under load. When I pulled my old 283 out to put in the ZZ4 I was surprised to find that both mounts had separated - there was nothing but gravity holding the engine in! I haven't noticed any increased vibration from them even though they are stiffer than rubber mounts.
