7 1/4 to 8 3/4 rear end swap info needed

You didn't say whether the Dart is a Demon/Duster body style or Swinger/Scamp/Valiant style body. If you can find a driveshaft from another body like yours that cane with a 318 (or 340) 727 and the 8 3/4 rear you won't have to cut your shaft.

The driveshaft is a different length on a Swinger/Scamp body style or the Demon/Duster shape, but both are somewhat common driveshafts to find used.

Most stock A body 8 3/4 rears came with the small bolt pattern axles. They are pretty much a bolt in swap with the larger spring plates and u bolts.