I've done the A-518 swap in my Cuda and kept the stock Slap-Stick shifter. Let me point one thing out: the lower pivot for the shifter linkage won't bolt to the tailshaft of the A-518 (like it does on the A-727). So you'll need to fabricate a pivot and find a good place to mount it. You'll see that in one of the links. Just thought I'd mention that up front and save you some reading.
