8.25 rear end swap in 66 dart

They stopped offering the 273 in 1969, so it must have been swapped in- could have been anything originally. Post a picture of the rear end so we can confirm it's a 8.25 (it could also be a big-bolt 7.25).
If it is a 8.25, it's a pretty straightforward swap, using the '73's shock plates and U bolts. You'll need to have the driveshaft shortened though, since the pinion nose is longer than your 7.25's.