The simple answer is; your 3-speed cross member can be modified to work with a 4-speed.I have attached pic’s of the mounting cross members for the 3-speed and auto. The 4-speed is either identical to the auto mount or so close that I do not know the difference. The 3-speed mount does not have the rubber isolation mount ends and the fixed ends would put a 4-speed too tight to the bottom of the floor pan. The3-speed mount in my pic has a strip of metal added to the ends that allowed the mount to be lower (further away from the floor pan) for a 4-speed trans.
Jim
