You'll need an adapter/spacer for the flywheel to use your old transmission unless you run a 99/2000 6.0L with the wide crankshaft flange. Check out www.hotrodlane.cc for some cool parts for the conversion. For the transmission I would suggest you get the internals upgraded to 4L65E parts with 5 gear planetaries and a Beast sunshell. Your third gear clutch set should be replaced with borg/warner clutches and steels from the 4L65E to handle the torque. You could just buy a 4L80e with the 6.0L engine and harness and be done with that part. You'll lose a few HP turning that big transmission but you'll have plenty transmission to do most anything you want with the engine.