Hello I know its been all most a year since you put this post on and wondering if you figured out what you wanted to know if not my my guess would be that you wouldnt have to do anything to the motor mounts but since your going to pull the motor i would go ahead and change them anyway and as far as the transmission mount i think you would have to change the mount and the bracket but if you had allready gotten everything to gather then what did you have to do because im going from a 400 to a 390 also but going to 4spd instead and let me know how well that 390 is i havent gotin yet but im wondering