From what my trans buddy told me a 518 will cost a small fortune to build and have live. Not many valve body choices for it neither. Could always put something else in it like a 4l80e.....but the purists will have some issues....
4l80e is double the weight and uses more hp to turn, the 904 soaks up 26 hp and the 727 soaks up 44.
The 518 needs the shaft drilled to increase oil to the od...
OR
don't race in OD or pull stuff and it'll be fine....
OR
Get a GV and have 6 spds to choose from....
Now putting the 518 in place of the 727 you need to cut the trans cross member and lower the mount.. i do not remember doing anything else.
Remember never leave the trans in park for long periods of idling...