Changing Drums to Disc

It sounds like you still have the 5X4 SBP wheels and hubs. The 73 and up have 5X4.5 LBP.
Something else to consider. You might be able to find a K-H 4 piston set up and keep your
SBP wheels

http://www.forabodiesonly.com/mopar/showthread.php?t=327732

Right here in the For Sale section of the forum.