DO NOT EVER USE A SOLID TRANS MOUNT, especially with solid motor mounts. You will break the trans case at some point. You have no more vibration from a 400 than any other SB if it is correctly balanced. Even full tube chassis cars have some flex, most of those use front motor plates and a mid plate. A factory chassis flexes more than you can imagine and even with a roll cage and subframe connectors if needed, it will flex enough to break parts.