"The car runs great and strong while out of gear."
If only we could harness this much power while IN gear, it'd be great.
Seriously, let's go even further back to basics. Superman covered some of them. Have you done a compression test? If okay, and there doesn't seem to be any excessive internal friction, then I'd suspect something in the basic setup or an adjustment while putting it back together (e.g. cam or ignition timing).
I suspect you haven't smelled burned friction material so I would rule out a slipping clutch. You do have free play? Partially collapsed exhaust system?
Difficult to diagnose via wire,
Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com