Want to put a magnum 5.9 in my 70 dart.

Since you said you've already gotten "conversion mounts", I'll ASSUME you currently have a slant 6, which in turn, would have an A904 if it's an automatic transmission.
Then your easiest swap would be with a '68 and later smallblock A904 (your slant 6 transmission will not fit the LA or Magnum engine block) transmission, which would allow ALL your existing shift linkage, speedometer drive, and driveshaft to swap right over to your new setup.
All you would need to fabricate or adapt would be your kickdown linkage, and oval out one flexplate hole on the 5.9 flexplate to fit the 904 convertor, as mentioned above. And the addition of an appropriate V8 radiator.
As I said, this will be dependent on if you're going to run the factory EFI or not.
Also see Magnumswap.com for additional info.
Seriously consider a heavier rear end (if the stock slant 6 7.25 rear), bigger brakes (if the stock 9" drums), and heavier front torsion bars.