I will assume by your name you have a small block,dont use an 18 spline,the 23 will be cheaper and easier to find a bellhousing,plus you dont need it.In fact you can do what i and many others here,get an 833OD out of a 80-90's dodge pickup,same tranny but 4th is overdrive.They have to pads for both B@ E body shifters,you may have to drill the pads and tap for the E body mount but thats easy.Its aluminum case which save wieght as well