What body are you planning on putting the 518 in? There are significant fitment issues with A bodies in the cross member area. E and B bodies aren't too bad.

If you want a non lock-up, they will be the early 2 pin ones. If the plug-in by the kick down and shift selector levers is a 3 pin, then the tranny is a lock-up.

The non lock-up 518s will use the 727 converter, the lock-ups cannot. And you will need the kick down linkage unless you use a manual valve body.

Do a search on Google on installing a 518. Plenty of info out there.