There are at least three manufacturers out there. Bill Reilly has been extremely helpful with information and support for swapping a 5.7 & 518 into my 72. The RMS AlterKtion looks to be the most stout of the three. It's about $3000 from any of them.
fastmopar
w/ motor mounts and power steering it ends up being $4,250 ](*,)
from what I've seen, heard and read his is the only one that bolts in first try nice and easy.