273 or 225?

Okay, if I'm reading this right, your proposed donor Dart is a wagon- Therefore you can't use the ball and trunnion driveshaft in your '66, since the wagon is built on the Plymouth 106" wheelbase and your Dart is a 111" wheelbase. The shaft will be too short- You'll need to have a new one built, if you're lucky enough to find a shop that will still do B&T shafts. There is a way to build a conversion shaft for the B&T trans., but we can cross that bridge later if you go that route.