Its not as straight forward as with a smallblock, the 518 was never built for a big block. You need cut off the bellhousing, and bolt on a aftermarket one designed for bigblock. There is a good thread on Moparts with a guy doing it for a truck.
You'd probably be better off with a good 727 and a Gearvendors overdrive. It will cost about the same if your buying everything from a dealer and having it installed. Its a bolt on (no fabricating) and you can take it off again if you wanted to. The reduction in labor alone is worth it, plus its supposed to be good up to 1200 HP. It also has a 2 yr warranty.
I have a friend with one in his Sebring and he loves it, it just works and he doesn't have to think about it.
I by the way, am installing a 518 in my Challenger....someday. I've had all the parts for years now, but its still not in and running.
Chris