Anybody know what length driveshaft I need?

Hey All

I've got a 318/904/7.25 Dart. I'm replacing the rear with an A body 8-3/4. Stock driveshaft is too long with the new rear end.

Also, how do I know which u joint the 8.75 takes? Rear end has a 489 case if that makes a difference.

Thanks!

Steve