Does a slant 6/904 drivetrain use the same length driveshaft as a 340/727 drivetrain in a 72 Dart Swinger? I am keeping the stock 7 1/4 rear diff for now.

I doubt it.

I would expect the 340 / 727 shaft to be quite a bit shorter.

Here's a handy reference page which runs down the proper way to measure drive shaft length for any application:

http://www.iedls.com/measure.html


---Red