My vote is also for a tranny out of a late Camaro or Firebird. If you get the crossmember, bellhousing,flywheel and clutch from the same car it'll be a LOT easier to install. The more related parts you bring home,the more you have to choose from when you start fitting it together and need to adapt something. Don't forget to replace the pilot bushing-buy one to fit the tranny you're installing. Send your ol' Muncie to me.
Speed