RE: switching from a 3 speed to a 4 speed?
My fathers 71 Cuda was also a 318 3-speed. Now is going to be a 440-6 18 spline 4-speed. Bought the trans for 1200 rebuilt, bought a new clutch bell housing clutch fork. Clutch pedals will work and hopefully you have the manual console because if not they are rather expensive used. E body Pistol grips are pricey also i cant quite remember what he paid for his i think 375. You probrally already have the Z bar. So its cost worth but money well spent in my mind.
Did your car come with 8 3/4 rear?