best 5x4.5 disc conversion on my 1968 dart 2dr post

Whats your price range?
Stuff like a distribution block/proportioning valve and new lines will be needed with any swap.
You can spend a couple hundred-ish on an FMJ swap or several times that for the aftermarket "kits".
Personally I'd go with the FMJ.
Oh wait...I already did :D